If the glass tape lies completely inside the fuselage, I would not bother to trim it down. If I had the option of going to the hobby shop and picking up a yard of light to medium cloth, I would cut a piece that extended the full width of the fuselage plus about 3/8 of an inch or so on each side of the fuselage to permit a more solid contact between the wing and fuselage.

It also prevents a lot of the denting that takes place on the wing between saddle and the wing when you tighten down the screws or use a lot of rubber bands for wing retention.