while in reality NTRP ratings seldom truly reflect the actual level of the player--in theory everyone rated a "XY" rating should be equal.
when you do lineups for USTA mixed leagues or Combo league, so long as the combination of 2 players equals the number of the final total-it isnt stacking.
So for example, say you have a bunch of great 3.0s and a handful of really good 2.5s, who should probally get bumped to 3.0 but anyways they are 2.5. then you have some weaker 2.5s
a 3.0 and 2.5 equals 5.5--that is all that matters.
Deliberate stacking would be putting 2 2.5s on the court as a 5.0 combination against a 5.5 team and the have actual 5.5 teams playing lower positions.