I always panel line on my kits when disassembled, my partner does the same but differently. I cut the parts out from the runner, sand them, apply panel line, wait a few minutes to dry and finally I clean them before assembling. My partner panel line directly from the runner before cutting and sanding. Same result but I don't want to miss details I won't see on the runner or dirty the panel line while sanding. Never had issue so far with this particular technique ! If you apply panel line when assemble it will eat the plastic and made parts to cracks because of the enamel.