That really depends.
To be honest, it's really easy to say it's all determined by destiny. After all, the Cutie Mark Crusaders could've been destined to meet, thus guaranteeing that they'd get the marks that they ended up with. We already know they ended up hiding together under a table in the very first episode and they didn't even know each other yet. And we know that even though none of the Mane Six had met each other as kids (save for Rainbow Dash and Fluttershy), that Rainbow Dash caused all of them to realize their special talent and gain their Cutie Marks.
I like to think that a pony has a choice when it comes to their cutie mark, but a big theme of the show is destiny so that wouldn't surprise me either.