I've checked in twice VERY early, but it was offseason so I knew they had a room available. I had called the night before and asked if I would be allowed to check in early if there was availability and they said yes. The second time the checkin guy gave me a weird look (probably 8am and any other person he saw was checking out). I just told him we checked in early last time and the manager gave him a nod that it was OK. If they have rooms available, there is no reason for them not to want to go ahead and get you checked in, one less person for them to worry about when it gets busier.