Yes, certain blocks of rooms are assigned/bought by travel agents to sell. That is why you may find that VH has no availability for the Poly on the 5th May, but Disney UK has rooms to choose from. No need to worry.
Xmas 2005 we wanted a room at Best Western Lakeside. Only problem was it was a last minute trip, didn't book the flights til 23rd Dec to fly Xmas morning. I checked every website I know, including direct they all said sold out, eventually came across one Fri 23/12/2005 10:41pm(just checked the email) on Travelocity, checked in on Xmas day no problem.
So no you should not worry
On the other hand in 1997 we turned up at a Marriott in Ft. Lauderdale after just driving up from Miami airport to be told they had overbooked and had no room for us They ended up offering us a night at their hotel at the airport then the next night back where we were supposed to be, for free.
Only time it has happened though, so while you can never be 100% sure, the chances are you'll be fine!