My company not only has negotiated corporate rates, but also blocked space rates. I am on a long term project, staying at the same hotel, and when ever I have trouble getting a room at our rate, I call the in-house reservations / sales office for assistance. I've never had an issue getting accomodated. However, I've stayed there nearly every week for the past 6 months. YMMV.

To be fair, even when I worked with a different company and stayed with a different chain, I've done the same thing and it's generally worked, unless there's a big conference in town or holidays.