Whether you cover the wing or the tip first make sure to leave yourself a good 1/4"-3/8" of material to cover over .Myself I do the tip first slitting the overlap then ironing it down to the wing then cover the wing .Trim it at the edge of the tip with a exacto knife or sharp sissiors then iron it down for a virtually seamless joint .Use the heat gun to shrink it and your done .
Hint get the covering as tight as you can when applying it and dont add excess amounts of heat to the tip where the seam is .Even a 1/2 overlap can pull if it gets to hot and also will create air bubbles that can show after it cools off.
This what I do for monokote, Ultracoat is about the same with allot less heat involved .
This I might add depending on the wing its a good idea to cover both sides and shrink both sides at once .do a little then switch to the other side and do a little this will help prevent any warpage or twists.