I use a Sennheiser EW 172 G2 in the band I play bass in and I've used it a couple times with my guitar. I never noticed any difference between running it with my guitar and just using cables. If you use a reasonably decent wireless setup you should be able to have minimal tone coloring. If you're set on buying one, I'd probably go to some big chain store like Guitar Center and just buy one, bring it back, try it, and if you feel that it colors your tone too much then return it and get something else. But it is important to ask yourself "Will I really use this?" If you just stand in one place when you play, then honestly, a wireless system is probably not going to be worth it. If you're playing on smaller stages where you can easily reach either side of the stage with a cord, its probably not a good idea to get one.... But if you're moving around a lot (going to different areas of the stage or even off of the stage), doing a lot of 'stage antics' or playing on larger stages, its definitely worth purchasing.
Lots of people say "Wireless kills your tone" but that's just BS if you ask me. A lot of bands that get amazing live tones use wireless systems and they're using practically the same gear that a lot of people on the forum here have. Just remember, if you buy some $89.95 wireless setup, chances are it'll color your tone negatively or just not work that well.