Are humidistats really the way to go with bathrooms? What if it's just to freshen the air after someone's used the toilet, presumably it wouldn't come on?
I'm open to them but normally install so the fan comes on with the lights on an overrun timer, but can also be turned off with a switch.
My home came fitted with both - it has its own separate switch beside the light switch to turn the fan on by itself (with an over-run timer) however, if you don't turn it on, the humidistat will turn the fan on when the humidity reaches a certain level.