I have always had a radio permit on board, and its part of preflight document checks for me as well. Ive always flown either 121 or 135, so this could have something to do with it as well. If youre purely part 91 or even 91K, you may be exempt. To be sure, probably check with your local FSDO. Those guys would love to answer your question. Better to know now and be sure vs playing dumb during a ramp check.