No sure... if I were to theorize about it might actually be something in the ROM itself... but I have never seen that issue... and I do flash a lot of ROMs... in the last month I've used Gingeritis 3D, a couple versions of CM7 and all the versions of BAMF Forever since 1.0.1 was released.
I'm not trying to say it does not happen, but I really don't think you need to flash a new radio, or the same radio, every time you flash a new ROM.... but if you do run into a problem then that is a good starting point.