There is less of a difference than you might think. Heck, with the addition of a little calcium chloride (to help bind the milk particles), you can even make cheese with skim milk. I flipped through Harold McGee's section on low-fat milk, and he doesn't give any warnings about using it in cooking.
Of course, keep in mind that your result won't have quite the same texture. I doubt you'll notice unless the milk is the primary ingredient in the sauce (and there's no other thickening agent) or primary liquid in the dough. If I was forced to come up with a way to adjust to using low-fat, I'd probably add a little more milk if I was going for flavor, or a little less if I was going for texture.