I don't think that in trench games there is a wrong time to receive spoils. Even Mid game spoils.
Because the game takes so long to play that any surprise assaults from troops received in Mid game spoils trade can also be seen from a mile away. Or at least the move before.
The perfect assault is in the timing but that is when your opponent have no spoils, then you can advance. But a mid game spoil set is always a welcome attribute for assaulting C from B on the next turn. That's about as surprising as it gets.
So no, I don't think that there is ever a wrong time not to receive spoils but there is a wrong time to trade them in for troops. What you don't want is to be so far behind in spoils where you only have one or two spoils on hand because then the opponents forces can advance in on you at your weak spots and you don't have a set to come back at them.
The perfect strategy in TW games includes always having the maximum number of spoils on hand before you trade them even if it means not attacking when you do have a set. In other words; If you only have 3 spoils and they are a set and you can attack then don't. Unless you are bound to win the game or some huge advantage.
Because if you trade in your set then you have no spoils. You end your turn and get one spoils and everyone can move in on you where ever you are at your weakest. So there is never a wrong time to get spoils but there is a wrong time to trade them in for troops.
I learned this the very hard way.
