The anodizing is meant to provide sufficient corrosion protection for the wing spars and carry-through. Painting the visible areas, or not, would be mainly for aesthetic considerations. In any case, there will also be portions of the spars and carry-through that are not visible, and are not accessible for painting even if you wanted to. So the anodizing better be sufficient in itself.
Remember though that if something does get scratched or significantly scuffed on the anodized surface, you will need to dress it and apply some form of corrosion protection over the area (i.e. primer and/or paint).
If you do decide to paint, surface prep would depend on the paint system you use. I used Krylon Fusion rattle cans, and all I did for prep on the anodized portions was to clean the surface thoroughly with acetone. I got very good results. YMMV with other paint systems. Whatever you do, keep this in mind: I don't think you'd would want to use anything etching or do anything else that could compromise the anodized surface layer of the spars. That is still probably your most important form of corrosion protection you have on those parts.
