They should really stay above 500 ft AGL, even though helicopters are conditionally exempted from the FAA minimum safe altitude rules if they need to go lower, but this seems to be one of the more grey areas that has not been clearly addressed. The assemblage of rules, guidance and advice that currently "regulates" aviation, including UAS, is not fully specified or even entirely self-consistent. It still seems to rely on the application of common sense in many cases.
The mosquito spraying near me takes place lower than 500' AGL. When I mentioned that I would be normally no higher than 200' AGL, they did indicate the potential for a conflict, and that's over my house/neighborhood, 3 miles or so from their pad. It doesn't happen all the time, but can and does.