I can think of two exceptions that support the rule:
Very impersonal transactions: say you're running a kiosk, and a guy comes up to buy a candy bar, and you recognize him from college, but know he's an asshole. You can still sell him that candy bar.
Smaller of two assholes: there will be times when you will have to deal with an asshole to avoid an even bigger asshole. Then it's fine; it's a life skill to be able to deal with difficult people.
Very impersonal transactions: say you're running a kiosk, and a guy comes up to buy a candy bar, and you recognize him from college, but know he's an asshole. You can still sell him that candy bar.
Smaller of two assholes: there will be times when you will have to deal with an asshole to avoid an even bigger asshole. Then it's fine; it's a life skill to be able to deal with difficult people.