wow! that really is a good question. a genuine head-scratcher. right up there with "if a tree falls in the woods and there's nobody there to hear it ... does it make a noise?"
and ... if it does get "dull" ... does that make it better? or worse?
i'm so glad i don't have any ballpoint needles. i'd end up spending my day playing with it to find out.
i've recently noticed that by the time my pointy needles need a change they've not only become dull, they've become ever-so-slightly crooked. i can't see it, but it's just enough to take the needle's eye out of line with my auto-threader. i was beginning to think my threader had stopped working and was really irritated about it because my machine is only a few months old. then i stuck in a new needle and it worked perfectly again.
my point being (sorry ... couldn't resist the pun) that you might want to watch for other types of needle wear - not only whether or not the tip is still permorming the way it's supposed to.