Charge dry and someone will lean in all the way out just to save a bit on fuel!
Actually this "cheating" must be a UK thing, because it has never every crossed my mind to speed up checks, or turn off the Master in flight just to save a few $$...Mind you, I suppose if you're paying £300 per hour + VAT I can see the temptation. When you pay $65 per hour for a WET 172 in the USA, there really is no point....