I had a yellow Tang which also did this but he died . I don't really think this is a good thing but I'm not an expert. I have a new YT and I don't ever see his white stripe. You have to extra care with this type of tang. They will die if you don't really take care of your water.
i have never seen a yellow tang that didnt have the white stripe. there is absolutely nothing wrong because of the stripe. every yellow tang at inland aquatics has them too. i go there at least once a week.