Oh, sometimes the fifth, sixth and seventh work too...
One of the dairies I worked at there was a heifer I called Breakfast - because I sneaked a feed off the fifth while the milking cluster milked out the other four. If she came in the wrong side to access that teat I just swapped one of the milking cups onto it when she was nearly finished.
Beef cow they'll probably never cause a problem unless they get mastitis. Dairy cows, they can get in the way of milking - I check the calves and snip them when they're little. The only thing to be concerned about breeding is that the heifer's calf is likely to have extras as well.