I think it all depends. If you are familiar with all the restaurants and have a plan, you can do pretty well booking online an hour before the phones open up (assuming the system isn't down of course) for reservations. If you need help picking or have a lot of reservations that need credit card guarantee calling is good.
I did a combination of both for our upcoming trip. I booked what I could online in the morning as soon as I could, then called later for what I couldn't get to work (F! dinner package) and had a CM book it for me. Worked out great.