Nationally Recognized Testing Laboratories or UL Certification necessary for USB device
NRTL certification is not a legal requirement, but may be a practical requirement in many situations.  Resellers may not be willing to carry the item if it is not certified to their satisfaction, and some large institutions may have policies to not buy uncertified devices. Also, having certification may help in liability cases where the device is alledged to have caused damage or injury.

The legal requirement is that the device meet the radiated and conducted (if it plugs into the wall) emission limits as set by the FCC in part 15 of their rules.  You are not required to get unintentional radiators tested.  It is simply your obligation to ensure they comply.  Getting the device tested is one way for you to know it complies, and will help matters later in the unlikely event there is a complaint, the FCC looks into it, and finds some of the devices to be illegally radiating.

If the device is a intentional radiator, then it must be certified by a FCC-approved test house, and will have to be assigned a FCC certification number.  Intentional radiator are taken seriously, so you really need to follow the rules in that case.