Making the webs square helps align them perpendicular left to right, but what about making them perpendicular when looking at the plane from the side, i.e. leaning forwards or backwards?
I align ribs over the plan or over gridlines to ensure they are straight fore to aft. As far as leaning forward or backward, any decent kit or plan will tell you how to build the wing with the ribs on the board. For flat bottom airfoils, the wing is built flat on the board. For symmetrical or semi-symmetrical wings one of two methods is used most commonly. Either a stick is placed under the ribs at the trailing edge so that the ribs align properly with the main spar flat on the board or the ribs have tabs on the trailing edge that align them with the main spar flat on the board. The tabs are removed after the wing is removed from the board. As far as ensuring the webs are perpendicular, I notch both the top and bottom spar and make sure I can get both spars glued in place before the glue on the webs sets up. The notches align the webs vertically.
