@Funboy we essentially build every car twice. Everything is fit prior to paint and we try and have all of our cutting and drilling and fitment done first. While things change and sometimes this doesn't fit our schedule, so we're very comfortable in drilling and cutting on painter car, I would not recommend that for anyone, especially someone who doesn't do it for a living.
Regarding the "prep" comment from others. I cannot express just how true that is. I get clients who get impatient waiting on paint and body and they always ask if it is painted yet. 95% of the paint job is the prep work. The big payoff and the satisfaction comes in the very end with color/clear/buff but the dirty work in primer and sealer is what makes the paint job show quality vs. average. I won't get into material quality, thats a whole added discussion, but it matters also.