I've seen it done both ways (even by the 'pro's'), but here's my take on the situation:

It's far easier to cut out the body, make the body post, exhaust, heatsink, and cooling holes whilst the body is clear and you can see what you're doing.

Obviously you can't make the body post holes until the body is cut and trimmed, and how on earth people align body post holes through a painted body amazes me.