+1 for what emcon5 said. Rifle shooting is rifle shooting IMHO. A good rifleman can shoot with a scope, open sights, receiver sights - what ever. Whether it's iron sights or a scope it's all about sight picture, breath control and trigger control.
I don't know if shooting with iron sights improves one's ability with a scoped rifle, but I do know that shooting more makes one a better shot.