If it's a chain hotel with an 800- central reservations number, you get a quote, make the reservation, then go into the hotel and ask for the rate for that night. If they quote you a higher rate than you got on the phone, you tell them you want the rate you reserved at and give them the reservation number. If they quote you a lower rate, you take it and say nothing about the reservation. If you are near the hotel and it is after 6PM or whatever their cancellation time may be, then you first ask at the front desk for the rate, and then if what they offered on the phone is better, you either tell the desk that or step away from the desk, call central reservations, and book it with your credit card details to pay for the room. (If you really want to be in the desk clerk's face, which isn't necessary, you can call central reservations in front of them.)

I've done this a few times and never had a problem. I recall once years ago before the advent of cell phones, I was quoted a high rate at the desk of the Holiday in SF's Chinatown and went out the front door a few feet to the corner pay phone to do it over the phone for a substantial savings.