Tar and commercial pruning paints containing tar should never be used. They cause die-back.
I find that white glue (Emler's, Will-Hold, etc) is an excellent seal for pruning cuts. The cuts seem to heal more quickly if sealed, especially heading cuts.
When I prune my peach, I reseal old cuts where the wood has dried and cracked, to keep water out of the cracks. This prevents rot.
When I prune my roses, any heading cut on a twig thicker than a soda straw is sealed. Otherwise, stem borers (solitary wasps making nests) will enter the cut and drill down 2-3 inches, weakening the branch.