Definitely acceptable! And thanks for asking, because I'll add it to the FAQ later since you make an excellent point.

It would probably be for the best if you put it in your sign-up just in case your writer doesn't click through to letters. (Mention it in both places and all bases are covered!) If someone writes your triggers anyway (and hopefully they won't) and you let us know, we will make sure you get another story that is trigger free.