Definitely once you sign the contract, you are bound by the stipulations within it. This is one of the major reasons I have stopped doing group events at Marriotts. Last year, I needed around 22 rooms for a couple of nights. They quoted me a price that was $5.00/night more expensive than than if I used the online tool AND they wanted me to sign that contract where I'm commited to those rooms. Using the online tool to get all the rooms allowed me to cancel (any or all if needed) up to 6:00pm day of arrival.

Their reasoning behind it was that if we do the group contract, they could block of the rooms all in the same wing of the hotel and I would be able to get more Marriott points as a meeting organizer. In the end, there were enough people with Marriott accounts so we were able to give 3 rooms credit/points for the stay to each of them and we were still able to get everyone in the same wing of the hotel. (I called the hotel a week beforehand and talked with the GM.)

So the moral of the story is, if you want to pay more and have a more restrictive contract, then by all means use the Marriott Meetings/Groups team to set you up. Otherwise, do it yourself, or as we did this year, take your group to a different hotel chain.