Be careful where you are using frequencies below 512 MHz. In larger Metro areas, one or more of the TV channels between 470 and 512 MHz are used for two-way radio systems for public safety (i.e., police and fire/rescue). Interference would be a very bad thing...
I would assume that the interference would be noticed by the user of the microphone first if they picked a channel in use by local public safety.
But yes, it's always best to check the FCC database and find out what is being used in your area before making and wireless microphone purchase.