I've done it all ways and haven't been able to notice too much of a difference. I've sprayed the Mopar paint on bare metal, straight over etching primer and over filler primer. I will say that the filler primer parts have probably held up the best but, not by enough that I would say it's worth it.

The etch primer I used was Dupli-Color rattle can and the filler primer was PPG that I gun sprayed. Probably not applies to apples here since any 2k product is going to be better than rattle can.