Silly me, I made a comment about a practical reality that applies to 99% of light aircraft flights and someone immediately jumps in with the 1% scenario. Yes in a standard Piper when the purpose of the flight is to conduct the spin manoever and when both occupants are both short and weigh more than 24O pounds the seat position might actually matter. For all the flights where you are not intending to do spins and the pilots are not inordinately fat and short/tall, or are in a Cessna.....or in other words ordinary loading with ordinary people to go out and do and ordinary flight ......the seat postion just does not matter. In fact I will go further and say in the weight and balance calculation it is the weight that is by far the most important and for most flights a balance calculation is not required. Instead chosing a CG location from a set of "CG for stock loadings" for 4 or 5 of the most common loading situations is more than adequate for almost all flights. On the odd occasion that an unusual situation comes up (eg 2 pax in the back but only one up front, a heavy baggage load, or somebody unusually light/heavy) than yes a proper balance calculation should be carried out.
I would suggest any loading where the seat position is the differnance between being in or out of the allowable CG range, is reason enough for a major rethink of the loading plan.