I would never start the Helicopter whilst not at the controls and strapped in (unless my life depended on it). However once everything is stabilized and locked in place I have been in many air work situations where I just gotta pee!! (or just stretch my old back!) so step down to do the necessary with headset in place. Shutting down for just a couple of minutes in high ambient temps. just never seemed appropriate. Also as Pitch link refers I have started up with an external battery with no ground crew so once after start checks done stepped down to disconnect. I have to say though that I've never done it in a PAX carrying op.
TS