You can cover first, then assemble, but I prefer to assemble first, then cover (But, do not hinge the rudder, elevators and ailerons until everything is covered)

The trick is to lay 3/8" wide strips to all of the corners (Like where the elevator meets the fuse) before covering.

The reason I like to cover last is this - Think of how the joint between the fin and fuse looks when you cover first. You can see where the fin is going into the fuse. If you cover AFTER they are assembled, that joint can be filled with wood filler and sanded. Now all you see is a covering seam which is almost invisible.

For ABS wingtips, sand, prime and paint them first. Cover the wing, and cover the wingtip as usual, you just don't have to cover the entire end cap.

Now punch a bazillion holes in the covering right at the last rib to give the glue something to grab on to, and glue the tips on.

I have an article that shows how to cover a wing that shows the 3/8" Strips and how they are applied. Give it a look, it has helped many people:

[link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=726]Covering Videos[/link]