I think this is probably very individual depending on your family's likes and dislikes and personal habits. I wanted to make sure that our first trip was "worry free" about money, so I (be ready for some planning OCD behavior) went to WDWIG (they have menus and prices), identified the places we would eat and also "likely" meals. For instance, for my 16 yr old, if a place has chicken strips she WILL order those. I included drinks and everything. I then added it up, added tax and then tip (I figured 20% in hopes that ALL our service will be great). Well, now that I have made a short story long, I have a very detailed food budget and since this was the kids Christmas gift I figured in 100 each for purchases. For us, that was the best way not only to plan, but to insure we had enough $$ saved.
Have fun planning...I had as much fun planning meals as I did everything else!
Anne