I agree with Fewnix in that the best strategy may be, not to skip, even if you have a good position, you should still card even if it is just for 4 troops. My logic being that of the timing of an engine. I know it may seem illogical to compare the set of spoils trade in this way but if you consider that after 6 sets of spoils you are in line for the 7th. And this is what I believe that Fewnix is referring to here with his math calculations.
The 6th trade in occurs on round 7 normally. Because usually the first round is the set up round and no one should be carding until round two. Providing any exceptions of course. So by round 6 everyone has their sets and on round 7 the sets get traded in. Follow me so far. I believe that this is what Fewnix is stating in other words. OK.
So now you have round 7 and 8 for 2 more additional cards because in round 7, even as you are trading in for 4 troops being the first player to trade in you are also collecting another spoils card. So by round 9 begins the second sets of spoils and if you have been playing correctly so far then this is where the first player to trade in, the same player that traded in for 4 troops will also be the first player receiving 20 troops and under the ideal conditions, this just could be enough for an elimination of just one weak player and the game could be then, over.
Most of these games are in fact over by round 9 and I have even had round 7 victories in non SoC games. Check out this game for example. Just click on the game numbers.
Game 10902804The point is that an engine is a precise piece of engineering and goes off when it goes off and when it skips then something is badly wrong with the timing. It may seem like a disadvantage to be the first one to trade in for only 4 troops but it also means that you will be the very first one to trade in for 20 troops and that could just mean the end of the game and the victory for you.