I have been helping a coworker plan their first trip to WDW. They made their room requests the tried and true way of adding them to their reservation when they booked. They did their online checkin at 54 days out, just because it was something to do.

They did not make any room requests during online checkin. At 49 days out, they wanted to change their reservation and added 2 days on the back end. There was no change fee to do that.