I think I understand what you mean. I bought my kit from another builder who had the ribs attached to the main spars so I did not see the spar in it's original state. My starting point was the rear spars and then the top skin so I did not notice a bow on the assembly. Perhaps as you progress, the assembly will straighten out and eliminate this bow. To some degree, my skins seemed to lay perfectly on top of the spars and ribs and I did not notice a problem with bowing. However, clecos do align everything as they need to be before riveting. Make sure they are on and tight.