I'm similar to Fred on this one. I try not to touch my tip as long as it doesn't turn glassy on me. If I've miscued a couple of times or it just doesn't seem to hold chalk as well as I'd like, I'll run some sandpaper over it to rough it back up, but that's about it.
That said, talking to Mika Immonen at the Mezz booth at SBE this past weekend, he pulled out a tip tool that he said he picked up in Japan. It was about a 2x3 inch piece of wood with a very fine microfile surface on one side. He took it out to rough up a tip on one of the demo cues, and I was thoroughly impressed. It roughed up the tip, but didn't seem to change the shape or take much material off at all. I never got the chance to ask him how I could get my hands on one. If anyone on here can hook me up, I'll buy one for you when I buy mine!