Shure ULXS4, basses tuned down 1/2 step, no problems!
Most wireless manufacturers publish the frequency response for their systems. Many guitar systems cut off well above the low end of even a 4-string bass. I've checked a few systems with a signal generator and scope and they all matched their published specs.
Another concern may be dynamic range. Systems that don't use some type of compression/expansion often run out of range between max signal before distortion and min signal to stay above the noise floor. But systems that do use comp/exp can often mess with your sound. You'll probably notice it most if you're a slap/pop type player. It's not quite as bad on the fingerstyle stuff.
That said, I use a good quality cord whenever possible, and use the wireless when I need the convenience. I would NEVER recommend recording through a wireless system!
(Message edited by lmiwa on November 24, 2008)