"I've got several extra feet of antenna wire on my car's cb antenna. Is it best to coil it, wad it, spread it around or snip the excess?"
Wad it up. Coiling it acts like a choke and you lose your transmitting output. The length is critical. Take a look here
The Ultimate Guide to 11 Meter CB Antennas
You also absolutely MUST get yourself a SWR meter and tune your antenna!!!!!! This can make all of the difference! You can have a crappy-ass CB radio from Wal Mart and it will work just as well as the top-of-the-line big dollar radio if the antenna is tuned to the specific radio as installed in the vehicle. The main thing is what is the maximum output of the radio.
All you need to get is the SWR meter and a pigtail to put it in-line, tune the antenna so you have close to a 1:1 match (or lower than a 1.50:1 match) with your radio, antenna and antenna feed line all installed as it is in the car.
Also, for CB's the best place for an antenna is in the center of the roof of a metal skin car (Corvettes need not apply) and grounded to the body. you can use a through-the-roof mount (which should ground right there where it goes through the roof) or a mag mount but ground the coax to the body of the car or you will screw it all up. I use an ohm meter to check for a good ground.
Mounting your CB antenna on the bumper will work as well, but it will be somewhat directional depending on where the mass of the "ground plane" (the car) is. If you mount your antenna on the trailer hitch the radiation will be biased to the rear of the car (the metal mass "ground plane" of the car drives the radiation through the antenna).
Nuff for now?