I would reserve a preferred room if you want a guaranteed close location. All of the 'islands' have bus stops, so you will never be far from there. Really, with the exception of Barbados and particularly Trinidad South, you are still fairly close to the food court/main area.
The fewer requests you have, the more likely you are to get them. Be sure they are already noted on your ressie and make sure they know which one(s) are the most important to you. The earlier you can check in on the day of your arrival, the more likely they will be able to work with you.
We have always sent a fax with our requests a few days prior and have managed to get what we asked for EVERY time. Some say do, some say don't, it may have helped, may not have, but it sure didn't hurt.