No strong preference here; they each have their pros and cons.
If I arrive in the morning, I don't spend the day thinking about my trip and I potentially maximize my first day there. However, I take a chance that the room isn't ready and I'm limited in what I can do until I can fully unpack. That usually means a trip to the casino... with resulting negative effects on the bankroll (usually).
If I arrive in the afternoon, the room is probably ready and there's still time to enjoy it. But the lines are the longest then.
If I arrive in the evening, I can save the vacation day at work and possibly save some $$$ that I would have thrown away had I arrived earlier. But there are limits on what else there is left to do in the day and the best rooms may all be handed out.