Hello!
I say no use of putting the primer on before you've got the seams all filled and sanded smooth.
As for the pre-shading - our dear Gino here doesn't care for it any... I like to have my panel lines a little darker, to show places where dirt would accumulate, in the recesses and around the rivets (which are mostly flush, anyhow), and where the paint would start cracking first - while the centers of the panels, nice flat surfaces would be easiest to scrub and would get the most sunlight - so some fading.
Please remember that some dirt from the exhaust would probably be visible on top of the tail boom and on it's sides - it's the combination of speed and rotor downwash that put the most of it there.
I hope it helps you, good luck with your build and have a nice day
Paweł