5 teats while not normal is not real rare either. Can you see if four are evenly spaced and look functional? Most of the ones that I have observed the fifth teat is smaller and does not look as functional as the other four even on a heifer that is 15 months old. I would not cull her at this point just for the one extra treat.
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.
I've got two beef cows with six working teats. I just culled two that had five teats (four working) , but it had nothing to do with their teats. The others are all four-teaters with maybe nothing more than some buttons in the back if that.
The structure of the udder and teats is way much more important.
The number of them, not so much.
Extra teats is generally no problem. With all the people that see these "posts", there will always be someone out there with the "exception to the rule".
We just worked our spring & fall calves thru the chute yesterday, & I checked all heifers & snipped off any extra teats. Only because we show cattle & sell show heifers. It's not drastically discounted in the show ring. Just not as "perfect".
You never want to "snip" one off after she has been bred.