That's not what I've heard from many people. I'd assemble the entire plane with everything and install the elevator and rudder servos last. Also install the battery last. Test balance the plane with the servos and battery just sitting in various positions and see where they need to be to get the CG at 6 3/4. I do this with all my planes to avoid having to do the install the tail feather servos twice. When you test balance the plane don't forget to connect the servo extensions which can weigh as much as a couple ounces each.
ORIGINAL: FallingWithStyle
A F90 with a Saito 100 will balance just by moving the battery around. You don't need a pull-pull rudder or putting the battery in the nose unless you use heavy (2oz) servos in the tail. Even then try moving the throttle servo up front first. The shorter linkage is better anyway.
Clark