I always prefer to paint before cutting out. The shell is more sturdy and when the shell is cut out before hand it can be nightmare to mask sometimes.
I do have people send them 'pre-cut' and they're harder to stand up when its 'dry time'. Plus, more risk of getting a thumb print in the paint too.
But, thats me. And everyons different.
I used to charge £5 to cut tem out for people after painting, but it takes too much time up now so don't offer the service.