I thought about this practicality-wise for a while before writing this.
In the first place, any Moltres will be able to switch into SR twice anyway because of lefties recovery, so the only time a SR number really matters is if you are switching into SR during SS. Or, in the rare cases where you are tanking just enough damage to reach the safety margin and switching out, both builds can lose exactly upwards to 191 points of HP, and survive the next entry into SR so there is really no diff there.
The significance of hitting a lefties number even on an SR weak Pokemon is that so long as you survive until the end of the turn, you already make up for the 1 point extra of SR damage, and you are getting 1 extra HP for every turn you stay in, and 2 extra HP for every turn you spend using Roost, which is more valuable than arbitrarily hitting a SR number imo.
Great that you brought up Amoonguss actually, because it is one of those examples where UU runs 252 HP, while OU arbitrarily run 248 HP for no reason, considering the sweeping statement of "the rest are placed into...".
Maybe SS is actually very prevalent in UU, or maybe Moltres doesn't stay on the field very often, I don't know. But if that is not the case, then 252 HP over 248 HP should be the preferred build. Similarly for other Pokemons who hit a lefties number with 252 HP that currently have 248 HP some reason.