The problen IMO is due to the fact that the geometries on the nipple are tiny compared to the rest of the map. Because the nipple stretches over a rounded area, any texture deviation that goes outside the area of the nipple geometry onto the breast will stretch. You can use just a nipple map and asign it seperately but you should stick to nipple skin tones that are are only slightly darker to the breast skin, as the geometry on this area is not really round. It's more of a blocky, hexagonal geometry. You could go further and totally remap the nipples much larger, simillar to what was done with 'Eve' (I think).