When I look at my corp rate for both my normal Marriott and Starwood hotels, they're 6PM day of arrival.

I notice the 72 hour thing on Hilton quite a bit.

Marriott has allowed me to cancel well past 6PM as well. During the freak blizzard back in October, they refunded a prepaid stay in New York City at 9PM. I was stuck in Jersey...60 miles from the hotel...they refunded it with no questions asked.