I've never worried about it unless they're in a bay. I also don't approach anyone unless the other bays are full. The exceptions are people I see on a regular basis who pull in to just sit in their car, or get out and smoke a cigarette, or adjust their stereo, or anything similar. I ask them once politely to not park in the bay if they aren't planning to wash, the next time they're told I'll have the police issue a criminal trespass warrant. So far I've only had to do that once.