I've done it a couple times. At least twice for airport IRROPS and a couple times roadtripping across the Midwest. Hilton points (regular award), a Marriott category certificate, and IHG points. Done them all on the day of travel, a couple immediately before checking in.

In the roadtripping scenario, just do some general searches on your target end-of-day destination to make sure a whole Interstate exit isn't sold out for some local rodeo or fair or whatever. Do this *before* you get tired!! It's one thing if you don't get your Hampton and have to slum it in a Motel 6 one night. Another thing entirely if the closest available room is another 60 miles down the Interstate in rural west Texas.