MinnFlyer, I disagree completely. Shear stress does decrease toward the wingtips.

What you said might be true if it were a concentrated load at the far end of a cantilever beam. However, the aerodynamic load on a wing (and to a lesser degree the weigh of the wing itself) is a not a concentrated point load, but a distributed one along the span of the wing. As such, there is practically no load at all at the wing tip. This distributed load causes a bending moment on the wing (i.e. cantilever bean) and it increases toward the wing root. The bend moment consists of two components:

1. Normal stress (tensile and compression) along the span of the wing. This is carried by the spars.

2. Shear stress, which translates to a force which seeks to move the top and bottom spars relative to each other. This is carried by the shear webs.

(BTW, there might also be a third stress component - torsion - if the loading is not even between LE and TE along the span, but we can ignore that for this discussion.)

Because this bending moment decreases progressively from wing root to tip, these two components, normal and shear, also decrease accordingly. Therefore, it is perfectly acceptable to use progressively thinner shear webs, or none at all, toward the wing tips.