This! If you've paid for a preferred location it would be safe to do online check in.
If you haven't, and there is a chance you'll request a room change, then don't do online check in. Online check in is pre made packets and room keys, you just pick up and go. No changes. Will they? Yes, but it is unfair to the people behind you I agree.
Adding your request and the reason why you need a good location on to your resort res. is good, and I would also send POR a fax, to room assignor restating your request, and reason why, about 4 days before check in