Do you place it on the driver's side, passenger's side or middle of the backseat?

We installed our car seat bases this weekend and tried to install them in the middle seats. Well, the latch system on our base doesn't reach to put it in the middle. We ended up putting the bases on the passenger sides of our cars.

Also...if you use the latch system, do you also use the seat belt? We used both the latch system and the seat belt.