Planet Hollywood is one restaurant that we go to every trip, my DD13 and DD17 always ask to go. We normally go for lunch or early dinner and we have never needed ADRs.
We've been twice without reservations.... once on Sunday lunch (12/06) once on Tuesday lunch (9/07). The place was @ 25% - 35% capacity, maybe and no ADR's needed.
The best time would be lunch, fewer people, can look around more, and no need for reservations! Take money! There are two gift shops and the food is not cheap. We left with 6 shirts and misc. souveniers (golf balls/magnets). My son and I got fahitas (sp.), my husband got a burger platter of some type, we all had soda, and shared an appetizer platter and lunch still cost us about $50.00 (over the free $15.00).
We had it scheduled on our DP in April, but when we went in 2005, we walked up and I think had a 15 minute wait if that....I would say unless you go during a peak time like Christmas or Easter, then you are OK without having reservations.
I went last Dec to use the voucher we hadn't used for our Sept trip. They took the old voucher with no problem. I went for lunch and the place was almost empty. Definitely no ADR needed that day.