My understanding is that having a reservation still won't guarantee your room is available for FD - they have a virtual "bucket" of FD rooms and when you go to get FD, they pull from that bucket. If it's empty, then it's not available, even if you already have a room reserved.
To add to that, if they see a bunch of rooms already booked without them having to give FD, then they just make the FD bucket smaller, thus fewer rooms available.
If you need that room, no matter what (for whatever reason) and want to make sure you have that room, then reserve. If you could deal with some other room, etc., then not reserving and waiting for the release is the better plan.
Hope I said that right (I'm trying to help and repeat what the experts have said). Thanks Joe/Smitch