I don't rotate. I just replace the rear.
That's kind of where I am now.... but the new version of the tires i run don't look the same, so I'd have to have a mismatched pair. The rear one is looking pretty sad. They got through a full race season (starting in Feb, including trainer time--a real good way to kill a tire). I might just change both and keep the front one as a backup. That means I can finally ditch my current backup tires which have some bigger problems, like a cut I'd have to cover with a folded dollar bill.

The only time I've put a front tire on the back is when I had to loan a tire to a friend for a while, and I didn't want to give him the one that was nearly worn out, so I gave him an old front tire. The reason I don't rotate is because I wouldn't put the worn rear on the front, and putting a slightly worn front on the rear just means i'll be replacing the rear even sooner. Otherwise, I might get close to 2 rear tire lives out of the front.