I always find it interesting that Hilton offers the accessible rooms (whatever they call them) at a lower rate.
If you do reserve it, make sure to put a note in your comments that you do not necessarialy need an accessible room, and if somebody does, please take it.
I've been given one several times as an upgrade, and I've reserved them when presented with them.
I've never felt I was taking it from somebody, indeed when given it at check in it was fairly obvious to me that somebody else wasn't getting a regular room.
Like anything else, Hilton I'm sure on many days has way more accessible rooms available then they do people using them. They offer them at a discount to get people to reserve them.