most older kits that I have built supplied pre cut balsa shear webs to run vertical.never had an in flight wing failure with  or with out them per design specs.for all practical purposes in most models in the under 10 pound class balsa should do fine.remember to build to fly not to crash so lighter is better.I suspect that there is a cost consideration also in using vertical shear webs as they would be easier to cut and lees waste from supplied sheets.If I remember correctly when I built my andrews H-ray in 1972 he had vertical shear webs between the spars but also what coud be a 2nd web running horizontal at the edge of the trailing edge sheeting in every bay