Stuart Cole wrote an answer about Petzl Tibloc Ultralight Emergency Ascender on July 8, 2012
The teeth on the Tibloc are fairly aggressive yes. However, if used *correctly*, you're fairly unlikely to see damage to your rope sheath, unless you're using it all the time (as any ascender will damage the rope over time).
The damage usually happens when the rope slips through the tibloc, so it tears the rope as it goes. Just make sure the tibloc has got a good solid 'bite' into the rope and it shouldn't slip.
The best bet is to practice using it.
Also, even in the worst case scenario, a damaged sheath is a small price to pay for getting your mate out of a crevasse.