When I was a child, despite my fathers best advice, I always assembled the model and then painted it. I did this because the building is the most fun part for me. The results in the end were extremely hit and miss!
So today I now paint the parts on the frame, then assemble. It pains me to do this as it can feel like a bit of a chore, but the results are worth it. Much cleaner crisper lines, with intricate parts properly covered in the right colour.