I seat and crimp in separate steps. For volume loads, I use plated bullets. They perform better when seating before crimping because they peel very easily. This is especially a problem with mixed brass of different lengths and thickness. Everything else is generally for precision. Seating then crimping is in order.
As far as re-sizing goes, my RCBS carbide die sizes pretty low on the brass. Cleans up the bit of Glock belly.