I buy those cheap 81/2x11 packs of solids when they go on sale. I use them a lot for matting and cards. And I just love the way they look all lined up. A beautiful rainbow of paper.
Other than that I buy solids when buying pp. I've also been known to run to J's or M's for ONE piece of paper that I need. (Ask me how many times I've actually gotten out of there with ONE piece of paper!)