If you subscribe to Touring Plans, they have a fax service where they will fax your requests 5 days ahead of your stay. They are still only requests.
If you have multiple requests (try to keep it to no more than 3), it's best to prioritize them however the request is made. That way the assigner knows what is most important to you and will look for that first, then see if any that meet priority 1 also meet priority 2, etc.
And yes, make sure that what you are requesting is what you have reserved if it is a bookable category. (Example, at Moderates, King Bed is a bookable category which could be in any section. So at CBR, you could have booked a King Bed - you could request it be in any of the sections (except Trinidad South since there are no King Bed Pirate Rooms) without it throwing a wrench into things. But if you've reserved a Water View room, it would throw a wrench into things to request a King Bed.)