I have much better luck cutting the slots before covering than cutting the slots after covering. I generally trim my covering to the trailing edge when I start. (I also start covering the bottom of the wing first) By doing it this way, I can locate the slots through the covering and not take a chance of messing up a nice covering job with the slip of the hand or a slip of the slot cover.

I would also like to compare notes with some of you other covering pros to see if my thinking is similar to yours.