What spokes wrenches are you using? If you are using some cheap multisided wrench, throw it out and buy some good wrenches designed to only fit one size nipple. That way you'll never accidentally use the wrong size. Park Tool makes a spoke wrench that grabs on all four sides of the nipple which should make it very difficult for you to strip a nipple. I've damaged a few myself by carelessly letting the wrench slide down near the bottom (towards the hub) of the nipple where it is not supported by internal threads (nipples are slightly counterbored). Push and hold the wrench as close to the rim as possible and you should run a very low risk of stripping a nipple using a quality wrench.