This probably depends on the season you are planning to go. I know that DTD does get busy in the summer nights. It wouldn't hurt to call Disney Dining and ask if a ressie is needed/recommended. You could always cancel if you change your mind.
I wanted to make ressies for my birthday dinner (they have a wonderful party atmosphere, can't think of a better place to celebrate), but was told by the Disney Dining folks that during the summer, they don't make ADRs until after 10:00 pm. A friend and I went last summer around 8:00, and had to wait almost 2 hours to get a seat.
Anyway, we decided just to make a late night of it. We are going to go see Transformers (My B-day in on 10 July) at the nice movie theater there, then dinner at PH at 10:15, and then on to Pleasure Island to celebrate the rest of the night.
There is also a site that offers a coupon for VIP seating where you get the first available seat, not sure how much of a wait that would be - here's the link... http://www.destinationcoupons.com/Florida/Orlando/dine_planet/coupons.asp
Last November 16thish, the wait was over an hour at 7pm - we had ADRs and were seated immediately. Never hurts to have an ADR.
Just a heads up I got one of the VIP coupons and they seem to put you in a different queue from everyone else, I waited 90 minutes with the VIP. I would try to get a reservation as its usually pretty packed.
I'll probably try the VIP coupon, 10:15 is pretty late for dinner. Like mentioned above, they don't make reservations till after 10:00 pm during the summer Maybe if I show them it's my birthday they will get me in earlier.