Clearly with all aspects of preparing an aircraft to depart its W & B is a very dynamic issue.
Hence, there is a need to use reasonably flexible loading systems so that the folk at the coal face can incorporate the weight control aspects into the main job of getting people and freight onto the aircraft and the aircraft away on time
... are all that relevant?
.. comes down to the need to put a line in the sand somewhere as to what level of accuracy is pertinent and important
Given the lack of precision in the W & B calculation
The calculations, per se, ought to be quite "accurate" as they involve only rudimentary arithmetic with little importance from round off and like errors.
However, given the sideline accuracies (errors) in things, such as,
(a) empty weight and CG data - there is little point in running the sums to ten decimal points worth of kg and mm if the starting point is, say, only accurate to 10-100 kg and 5-20 mm, depending on the aircraft size. Having done a lot of work on weighing errors, my view is that anyone who suggests that the empty weight (and, hence, the final calculated value) is accurate to the kg and mm is not aware of the realities of weighing and weighing equipment.
(b) standard weights - providing that the passenger load is moderate and the passenger population reasonably approximates the statistical basis for the standard weights, the use of standard weights is reasonable. Otherwise, all bets are off.
(c) use of loading zones - if each loading station is calculated separately, then the final errors are constrained and managed easily. With loading zones (a practical necessity for larger aircraft) it is quite easy to exacerbate loading problems unless there be prescriptive procedural requirements (and this, I suggest, is a typical reason for the CC's requests to passengers regarding reseating)
and the significant adverse consequences of serious misloading, one needs to be a little cautious in how one approaches the general loading problem.
.. moving one passenger could inadvertently cause a marginal reduction in envelope margin rather than the desired improvement.
.. probably not a concern of sufficient import to cause one to loose any sleep
a full and precise W & B calculation check should be done after each change in load including knowing exact weights and loading datum arms of all cargo, passengers, fuel loading, etc.
No problem doing that. However, the statement is not entirely correct or necessary. More importantly, the loading change infers that the previous calculation is now in error to some additional extent and that error needs to be looked at. There is a variety of techniques to do that - only one of which involves a complete recalculation of the original loading system.