One carrier says no more...
"Do not make any modifications to the tractor interior or exterior, including additional lights or stereo speakers, drilling holes, colored lens covers, decals, AC/DC converters. Decals of any kind or personalized plates are not allowed for the interior or exterior of the truck with the exception of a "4X6" American flag. Any unauthorized modifications will be reversed or removed at the driver's expense via payroll deduction. Many decals do not project a professional image. Offensive decals have become increasingly widespread, and many decals are poorly done - CB handles being the #1 example - and they simply don't look good. And the same can be said for multiple personalized license plates. Offensive or aggressive "CB Handles" can also cause liability issues in a cases of preventable or even non-preventable accidents."