In my experience, an early check-in is a non-issue. I don't even bother requesting one when I make a reservation. In the past several years, I've checked in as early as 8am. Literally, the only times I have had any issues is when the hotel gives me a great upgrade and the room isn't ready. When that happens, I have been given another room and a phone call when the better room is ready. Not exactly something to complain about.