Book as soon as possible. The best deals are usually in advance, unless the cruise doesn't book full and you are one of the special categories that will get offered discounts (Disney CM, Florida Resident are usually the first, they seldom will need to offer discounts beyond that).
There are only 60 category 8 staterooms on board, more than Navigators verandas, but I'd book early. They are all (if I'm reading the passporter right) forward. If you have kids, you might want to try for deck 5 (same deck as the lab/club. Deck 6 will give you a deck between yourselves and the Walt Disney Theatre (rooms don't tend to be noisy, but I might do this anyway. Deck 7 closer to the pool.
Forward tends to hold the adult areas - the clubs, spa and adult pool are all forward. Restaurants and the Mickey Pool are aft.
I wouldn't hesitate to go for a room close to the elevators. We ended up there with little choice and I was worried about the noise (aft elevators in our case). We booked it again because I found the hallways tight and the stairs/elevators pretty quiet.