I replace them. I find that if I stock up on tubes when they are on sale, it is well worth the cost to not have to chance it. In the past I used to ride patched tubes and never had any problems until one day.... I suspect you know how the story goes. When I'm on a ride and flat, I may repair the flat by patching the tube (if I have enough time), or I may simply throw in the spare tube I carry. I have learned from experience, however, to hold onto the first tube until I get home. It's no fun putting in a new tube and five miles later blowing the same tire again only this time the tube can't be patched, and not having the original to patch.