On-line booking does not usually give you the option of a day rate, or indeed an early morning arrival. I find that it's best to e-mail the hotel or to call a couple of days beforehand to advise them of arrival times, especially at airport locations.
Yes make sure they mail you conformation. It helps not to have an argument at 6am on arrival with a check in clerk who insists you are arriving after 3pm as happened at LHR with myself recently. Fortunately, I had recorded the call, and it turned out that the a-hole check in clerk was the one who took my call!