I arrived at my hotel at 3pm last month, they checked me with minimal delays. I went by the lobby at 4pm (official check-in time). The line was 10 times longer than when I checked in.
Here's another thought.... are you planning dinner at a popular buffet (eg.... Caesars' Bacchanal)? Consider an early dinner like 4:15pm to avoid the seriously long lines later in the evening.
Arriving by plane? There's savings travelling at certain times of the day.