the natural fuel burn-off during flight usually, but not always, makes the W & B envelope progressively easier to meet as the flight approaches landing.
While noting your caveat, the underlying generalisation doesn't generally follow.
Usual practice is to check correct loading at TOW (or ramp weight) and ZFW. These two points usually cover most bases.
However, it is quite possible for some aircraft to start inside the envelope at TOW, move outside the envelope during flight, and then come back inside towards ZFW (or LW). There is no a priori reason why loading gets easier (or harder) as the fuel burns down.
I also find it unbelievable that with the majority of airlines assigning passengers to specific seats during check-in that it appears to be beyond the wit of man to build the W & B calculations into the seat assignment software!!
Not the case. Most systems will be driven by the desire to achieve the optimum cruise CG. While this often creates some interesting difficulties for light loads, the aircraft configuration normally will be associated with the operator's mean loads.