On most of todays aircraft, the CG will move toward the inside of the W/B envelope as fuel is burned, so moving pax may be required to move your takeoff CG into legal limits for T/O, but once airborne it usually doesn't matter where the pax sit. Draw a line between your takeoff CG and your ZFW CG, and you will probably see that it cuts through the interior of the box as you burn fuel. I'm sure there may be different aircraft out there that are more CG sensitive, but for a modern passenger transport, once airborne, the CG will most likely be moving in a more stable direction. Aft CG's can adversly effect VMCa after engine loss (almost negligible in light aircraft - increasing with heavier machines), and on longer or stretched acft, increase chances for a tail strike.
Just my 2 cents. If you are really concerned, save a copy of the WB form and redo it with the pax in the original sections and a landing fuel figure. It should fall on the line described earlier.
Fly safe, all.