And if one does, is it bad form to then select one's own answer as the preferred one?
Not bad form at all.
The way you choose the accepted answer is your own personal choice. You also don't need to choose any if you don't want to, and you can submit and choose your own answer if you want.
I would suggest only choosing your own answer as the accepted answer if none of the other answers have helped you with the problem, not even a little bit. But that is just my suggestion, and it is really up to you what you do.
The only exception to this is that you shouldn't use self-answers as a way of spamming this site :)
Edit: as others have pointed out, let's clarify something: answering your own question is for when you didn't know the answer at the start but you subsequently figure out an answer later. If you knew the answer all along, you probably shouldn't have asked the question. But then, opinions will vary on whether asking and answering it anyway will overall be a net benefit to the site.