Most of the time it appears as if it's leaking before the nipple because pressure from radiator actually forces coolant back up towards the tank and leaks out underneath appearing to be a leak from under the tank. It doesn't actually matter to me one way or the other-just trying to be helpful as I've got a few years of experience with these things. Back in the 90's, I would have people bring these things back telling me they are leaking and I would take a blade to them right at the counter and send them on their way. If I had a newer guy at my parts counter, he didn't always remember to do it before the customer received it. The only ones I've ever seen that did not have some sort of flaw in that area are OEM tanks that were originally installed in the vehicle, and on those if you look close you can generally see where someone cleaned them up.