Because of my flight or drive time, I usually check in around 11am. All, except one has the attitude that if the room is available at the time you're there, then it's your. All with the exception of Caesar's Palace waive the early check in fee, or at least for me.
Here's the list of hotels where they didn't bitch about my early check in if the room was available.
Aria
Bellagio
Mirage- although I had to wait but they felt bad so they upgraded me to a suite for free.
MGM
Encore
Wynn
Flamingo- They had me wait, almost like they're trying to torture me so fuck them. Never again!
Mandalay Bay
Venetian..I think I got lucky there.
And the one resort that charges an early check fee, regardless if the room is ready or not
Caesar's Palace-So screw them now and forever...hope they rot in hell!!!
Hope this helps and remember..YMMV.....LO

Hi,
   I work at one of the hotels/casinos in Laughlin. For us, we dont charge a fee for a guest to c/i early. However, if they're requesting a specific room/floor and dont want to another room, they'll usually have to wait until that rm is clean. Depending on how busy we are, usually we'll try to put a guest in a similar type of room if its clean already if they arrive early so you may want to ask them if they can do so next time you arrive there early.