I've only wiped Hoovey off with my hand, and he does start to threaten. A corrective slap on his butt and he stops, so I think as long as I have someone there the first time to watch in case I miss him warning, he shouldn't be much of a problem.
I know my riding instructor does them as they need it, which is usually twice a year. They use Tuck's Pads on the mare's udders and bits, which are a human female bits cleaning product.
I'll probably end up cleaning Hoover when ever he gets too dirty down there, but I can't really answer the poll yet.