I just replaced my rear tire. I had noticed a couple of small cuts in it, so I asked at the LBS if I should replace it. They said the cuts were not such a big deal, but it was showing wear because it had become more squared-off than round.

I bought a new tire but it took a few weeks for me to put it on the bike -- in the interim I noticed that the bike felt unstable in back during left turns. I finally put the new tire on over this past weekend that solved the problem.