I always apologize with the hand held up and often even verbally say something as well. I will even do this if my opponent still wins the point after I hit the tape or framed the shot. I know I am pretty unusual in that regard, but I guess the motivation for that comes from me feeling bad for my opponent having to deal with that weird ball and also from my disdain in myself for the mishit. In the end, though, it mostly balances out with both players getting favorable luck here and there so it's not that big of a deal.
On a related note, though, I do have a guy I hit with regularly who always seems to come up with a frame lob or net cord pass on crucial points deep into a set. We both laugh it off, of course, but it really does happen more often than not in tight sets.
Descartes once said "Cogito ergo sum" or "I think, therefore I am."
Based on that, I don't think he would like this forum very much.