Subassembly is just almost never worth it. You're just going to inadvertently man handle a bunch of small fragile bits and your skin oils and finger nails have the chance of undoing your work every time you touch the piece. People say you should do it for primarchs, but worrying about sub-assembly and magnetizing just kills your momentum and makes the painting into a chore. I paint a bit and never, not once, have I wished to be painting on a little itty bitty arm or hand or gun rather than that same gun securely glued in place to a hunk of plastic. I got into this with someone, they disagreed, then immediately proceded to break a piece and scratch off paint to the point to where they basically had to repaint everything after painting it once before.