Don't use thinner wood in the wings to save weight. False economy and it'll bite you.

Do use contest grade for the sheeting. Use contest C-grain for the ribs.

And somebody suggested weighing the wood. Excellent, excellent idea. For two reasons.... well, maybe three depending on how you count.

Weighing lets you choose lighter wood. It also keeps you from using too light a wood. Very often the lightest wood of a type/size is actually the same false economy as using too thin wood. The other reason to weigh is a bit more subtle. You want the wing to balance spanwise, right. Use the heavier sheet from the root out. It's also the stronger sheet. Use your lighter sheet toward the tips. And when you weigh your wood, you insure that you don't wind up with all the heavy wood in one side of the wing.