I've found booking online and then submitting a separate e-mail has been fine for getting confirmation on what upgrades will be given at a particular hotel. I've been pretty satisfied with most of the upgrades I've received since becoming an RA, though it helps staying at IC's with plenty of better rooms to get upgraded into and a small number of room categories overall. And just keep in mind, even if you don't get quite the room you were hoping for, you still get to drown your sorrows with the contents of the mini-bar regardless.