Don't forget that the management of each hotel aims to keep their employees happy. A van driver that doesn't get tipped will complain about those cheap crewmembers who won't tip, and who knowing a specific airlines track record will provide good service or slow service.which can affect the hotels decision as to whether it's worth it to keep the contract.
if you hate the hotel, go ahead and be cheap. If you like the hotel it's in your best interest to do the societal norm.