If they are starting to tear, get a backup pair broken in soon! You never know when they're going to break. But as long as you feel safe in them, I suppose you can keep using them. If you've got a lot of small tears already, though, you might want to retire this pair so you don't have them fully rip on you during the middle of a skill. I like to "rough up" my grips and after a while they get so flattened that I can't rough them up anymore and that's when I get new grips. Most people keep their grips longer than that, though.