I don't touch vents, never had a complaint over this. I'm just not that fussy I suppose. You know people retain window cleaners for reasons other than purely clean windows. We're friendly, we don't rob them, we provide a good service, we charge a fair price that's both fair on the customer and to us, we're not too regular, we friendly, we offer a range of payment methods, and I'm good looking.
Maybe they'll weigh all that up and think a few spots or runs every now 'n' again is a fair one?
But I don't intend to start making problems for myself by cleaning vents when there's no problem with us not cleaning 'em in the first place.