I've got an ATV3, late 2008 Macbook and Time Capsule (1st gen). Streaming works fine even when the Macbook is asleep as long as the lid is even slightly open. All the WOL boxes are checked. Doesn't bother me so much as I just leave the Macbook open.
I don't have an Apple TV but I stream music to an Airport Express via the Remote app. I managed to wake the computer with the lid closed with the Remote app but it only stays awake for a short time (like 10 seconds). It then goes to sleep again. If I leave the lid open it's no problems to wake up the computer and it keeps awake as it should.
I don't know if it's a bug or if it's supposed to only work with the lid open.