Mdollar,
There's often confusion between pre-booking seating together, and Your Seat, Your Choice. Pre-booked seating together just means seats will be allocated together for all those under your booking. This can include seats across an isle or one behind the other. If you want to choose a specific seat number, this is at extra cost on top of pre-booked seating. Pre-booked seating alone doesn't allow you to choose the seat numbers. If you just want to guarantee seating together, pre-booked seating alone is fine. If you want to sit in a particular seat, eg I wanted a particular window seat the other week, then some choose to select a specific seat number. If you aren't fussed where you sit though, just together, then don't worry about selecting your seat and save the money.
Darren