Autonomous Access Points (APs) can send out BPDUs and participate in STP. There is the potential that the AP BPDU may have a better BID than the current Root Bridge. In that case, applying Root Guard to the port connecting to the AP would protect you...

Thanks for bringing this question forward! We have published new documentation on STP guard configuration that incorporates STP guard recommendations. Check it out and let us know what you think!