Does anybody use wireless with a 5 string? If so what model wireless and does it fall off below a certain frequency or note?

I use a shure pgx system, no problems. I also tune down a 1/2 step.

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)

Sennheiser ew100, never had any problems.

I use the Shure systtem as well, it works great

The X2 or the original X-Wire.  Same technology - same huge dynamic range (10 Hz - 20 kHz) with no compression.
 
I had an original X-Wire and A/B'd it with the new X2.  There is no difference.
 
I highly recommend the X2.

I suggest Sennheiser or Shure. I use a Sennheiser. I play everything from small venues to large outdoor gigs and have never had any problems.

Use a digital system like the X2, noise free, no compression, no loss...
 
(Message edited by Mario Farufyno on November 25, 2008)