My first instinct would be to build first, sand later...because if you put any dinges on the ribs during building, you can sand them out when its built.....and if u sanded the ribs first, u wouldn't really know how far you were sanding them...and you might risk sanding away too much...
I'm no expert, I'm still on my first kit-built plane...so maybe someone more experienced might be able to advise!!!....but my wing worked out ok doing it my way...