The reasoning....
Painting before assembly is GREAT if you have a LOT of patience... because you are going to need to double paint, and double bake your rifle. If you have that patience? Go for it - it's the very best thing to do,
Paint inside receiver (trunion contact points), outside trunnions, barrel tail (ever notice how rusty these get when you pull 'em?), Outside receiver under scope rail, BACK of scope rail, UNDER trigger guard points, OUTSIDE trigger guard points... you get the idea. Bake - assemble, then get ready to refinish :) Yup, you're gonna scratch something :)
Now - if you just blue the contact areas... Blue is not as good. BUT, after you refinsih & bake, you are going to slather those areas with CLP. Let it creep in. If it can not creep in? Neither can water. Not unless you shower with it :)
I'll admit - it's not as good as a twice baked AK - but its pretty damn good with a good kit. Observe the contact areas of your kit - were they painted? Generally, on the GOOD kits, they were just parked, then riveted.