We've talked before about why decreases lean, and mirrored decreases, but there's one thing I didn't get into back then, and that's the niggling little way that our left-leaning decreases never look quite as good as our right ones. Those k2togs are nice and crisp, but SSK, SKP... they're tough, and usually the top stitch of the decrease ends up being much more prominent than the top stitch of the right-leaning decrease, which is annoying when you're trying to do them in pairs. I'm hardly the first to note this; for instance, a while back Nona discussed this on her blog, and came up with a comparison of several left-leaning decreases. I touched on this while talking about neater ribbing, but I thought I'd go into it in a bit more depth; I don't necessarily have a conclusion here, but I do have some more thoughts on why we see this phenomenon, and some ideas for you to explore to address it in your own knitting.
First, why does this happen? At first glance, there doesn't seem to be a structural reason for it; a loop drawn left-to-right through two stitches with their right legs in back (an SSK) ought to produce the exact mirror of a loop drawn right-to-left through two stitches with their left legs in back (a k2tog). That leads to the idea that maybe the difference is executory, and perhaps is caused by putting too much tension on the loops when they're being slipped and/or passed over. However, if that's the case, there's an obvious solution; on the prior row, wrap the two stitches the opposite way so they're already sitting with their right legs in back, and you won't have to do any slipping. It turns out, though, that while this certainly does speed up the decrease, it really doesn't do as much as you might expect for the overall look of it. This surprising result got me looking more closely into the mechanics of forming the decrease.
It turns out that there is a structural difference in the formation of left and right decreases, after all. June Hemmons Hiatt, in her illustrious The Principles of Knitting, pointed me in what I think is the right direction, when she noted that the rightmost stitch of a 2-stitch decrease will always be bigger, and that the reason the k2tog is neater is that the bigger stitch is hidden under the other one, whereas in a left-leaning decrease it's necessarily on top. That got me thinking about why it would be bigger, and therefore looking into exactly what really happens to the stitches as they're worked. Work some plain knit stitches, and take a look at how the yarn moves as you complete one stitch and pull the needles apart. You'll notice that as you separate the needles to insert the tip of the right one into the next stitch, the strand between the stitch you've just worked into and the one you're about to work is pulled a bit and that this tugs on these two stitches; it's not much, but it actually pulls the one on the left slightly open, and the one on the right closed. When working even, you don't notice this, because each stitch goes through the process from both directions -- enlarged when it's the next one up, and then snugged down again when it's finished. In a decrease, though, the rightmost stitch of the decrease goes through the first half of the process, pulling it open, but it gets insulated from the latter part where it ought to get snugged down again -- the leftmost one gets snugged, but this one doesn't. This explains, too, while the first-blush solution of avoiding the need to slip can kind of work, if you do it carefully -- if you work it up on the tips, you can minimize the "pulling open" part of the process -- but mostly doesn't, because you still miss out on the following tug. It even fits with the fact that some people get better results with an SKP than with an SSK -- if you just slip the first stitch, then it does get tugged down when you separate the needles to knit the second one; you've got to open it back up a bit to get it over the knit stitch and off the needle, but if you're careful you can minimize that.
A very careful SKP is one at least partial solution. Ms. Hiatt offers a couple of suggestions: if one is knitting flat and the decreases are fairly separated, she suggests using a SSP (slip 2 stitches individually knitwise, return to left needle, purl together through the back) decrease on the WS instead, so that the stitch which ends up on top on the RS is the second stitch in the decrease, which gets its tug in due course; if the decreases are adjacent, so that the 1-row deviation would be apparent, or one is working in the round so that the WS is not available, she suggests achieving a match by loosening the right-leaning decrease rather than tightening the left-leaning one, using the technique of knitting a stitch, returning it to the left needle, passing the following stitch over, and then returning the worked stitch to the right needle (one may abbreviate this as k-return-pass-return, or KRPR), and I'll add the additional step of being very careful and delicate when working the following stitch, so that that top stitch doesn't get tugged down too much and defeat the purpose of these maneuverings. And then there's the so-called "improved" SSK, where one slips the first stitch knitwise but the second purlwise, and then knits both through the back; this twists the second stitch, which has the effect of tightening it and making it pull slightly on the first stitch, thus giving it at least some of the tug it would otherwise miss out on. I'm afraid this one makes me twitch -- it's clever, but it leaves a twisted stitch sitting there. Yes, there, right there, can't you see it? Well, I can. Ugh. But if you can live with it, since it is, admittedly, mostly tucked up under the other stitch of the decrease anyway, well, it's your knitting.
And then there are "brute force" solutions, which are just to tug that excess yarn back out of the problem stitch. At the time of doing the decrease, sometimes I will just use my fingernails to grab the back of the second stitch and give a little yank on it, which pops the first stitch into place nicely. By pairing this with the KRPR right-leaning decrease, I get quite nice matching, even in inelastic fibers like cotton; in wool, a careful SKP may do it, especially after blocking. If there's any looseness remaining, I've been known to use a crochet hook to grab the bar between the loose stitch and its neighbor to the right, and just pull it a bit, which gets the looseness into the bar, hiding on the back of the work, instead of in the stitch on the front. It's not elegant, but it is effective.
I'd certainly suggest that you try out a few of these on your own knitting, to learn what works for you; although there are some structural effects, execution definitely plays a large part here, and that means that individual idiosyncracies come into play. You may find, as I do, that a combination of techniques gives you the best overall result, or you may find that one technique is a homerun for you, and in any case, you'll learn a lot about how your knitting works.