You cannot make dinner reservations until you arrive at the resort. Others have posted that there are exceptions to this rule (e.g., you are planning a special occasion dinner), but for the most part it holds true. You can also reserve a beach dinner prior to arrival, but that is extra money.
It is really up to you, the resort and how busy it is. If there is a restaurant you definitely want to get into on a particular night, for us 8 Rivers, Friday Night for Lobster, then make your reservations when you arrive. If it doesn't matter, then wait and see what floats your boat.