In my experience (737) very little effort goes into deciding how a part-load of pax is sat. Generally it is 'stick them in the middle' which will not adversely affect balance, but it should be remembered that baggage loading may disrupt that 'careful plan'. Part loads can be tricky to seat, but since there is an ever-present ability of check-in/dispatch to seat the wrong pax at overwings/emergency exits (children, disabled, etc), I do not think we will see any 'logic' in loading part-loads. Remember that by the time it gets to producing a 'balance' on the load-sheet the seat allocation is well past, and trying to re-arrange that causes upheaval.