If you want the best covering job my mentor suggests covering before assembling. Then the covering seams can be finished off with epoxy the dries clear (not finishing resin that dries amber) and an "alcohol wet" finger tip. This will ensure that the seams are fuel proof. Also, there won't be a multi-layer seam.

I happen to prefer gluing up the horizontal stab & fin before covering so that a complete pre-covering assembly can be performed to ensure that all of the control rods, control horns, servo installation, radio installation, control surface setup and lateral balancing, etc. can be completed before covering all of the guts with film. I manage to catch a control / former interference (or something) during this test assembly. Once it's completed, cover with confidence that the final post-covering assembly will go like clockwork.

Good luck and show off your work!