Without the aid of pictures and a good look at your trees, I would not suggest any pruning unless advised by a professional. I say this because most conifers (pines, firs, spruces, etc.) usually exhibit sparse foliage near the apex of the tree because it is the youngest wood/tissue/foliage. This "sparseness" you mention toward the top of the tree is probably a physiological artifact of normal growth...which of course is a good thing!
If you truly think the "sparseness" is not an artifact of good growth, then please contact your local Extension or DNR forestry office for an on-site assessment.
Best of luck!