Definitely time for new ones if they become nicked, for safety reasons also. I was cutting along last week, and the rotary cutter hit a ruler nick and the cutter jumped the ruler, narrowly missing hitting one of my fingers. It wasn't even a visually noticeable nick, so I didn't think it was a big deal, but that scared me, and I went out the next day and bought 2 new rulers. I've had more than my share of quilting injuries. :O)