Nobody knows for sure, but the general consensus seems to be that as you get into the low 90's, performance starts to get affected, and the risk of injury increases. Not sure if anybody has ever been able to collect any real hard data to back those two things up.
During the season, I try to keep my position players at 98 or above. Though down the stretch, if you have to fight for a playoff spot, you might want to start pushing your guys harder to keep them in the lineup if they are your best options.