During low crowd times it is true that a reservation may not be necessary, but it can't hurt. And it can guarantee you a seat if there is a sudden rush when the entertainment starts, which happens on occasion.
During busy times, if you want to be seated during the entertainment a reservation is pretty necessary.
July 2007 I can say it was absolutely necessary. Early Dec 2007 it was probably necessary, although we we able to walk in to sit at the bar without issue.
Getting an ADR for a meal you WANT never hurts, unless for some reason you feel there is a good chance you will not be able to make it. Some ADRs come with a penalty for not canceling with enough notice (RR is not one AFAIK).