If you book early enough, you can book a specific cabin, although not in all grades. Once those allocated cabins are filled, they will default to guarantees so they can be more flexible in how they sell the remaining cabins. Even then, get your T/A or PCP to shake the tree for you.
No big deal. If you book late, all of the preferred cabins are already taken, so you may as well settle for pot luck anyway. Be wary though. Most T/As will pitch you their best price, and those will be on bookings with non-refundable deposits for guarantee cabins. There will often be specific cabins, but at a price.