There has never, to my knowledge, been a definitive legal judgement as to what constitutes "minimum safe altitude" for non-commercial radio controlled aircraft.
"While the FAA suggested that model aircraft operators fly below 400 feet, the document was just an advisory, and two aviation lawyers contacted by Gigaom said the agency has no authority below 700 feet — for now"
...and from the FAA itself:
"Flying model aircraft solely for hobby or recreational reasons does not require FAA approval. However, hobbyists are advised to operate their aircraft in accordance with the agency's model aircraft guidelines (see Advisory Circular 91-57). In the FAA Modernization and Reform Act of 2012 (Public Law 112-95, Sec 336), Congress exempted model aircraft from new rules or regulations provided the aircraft are operated "in accordance with a community-based set of safety guidelines and within the programming of a nationwide community-based organization."