When do you use さん after a occupation? For example, would you say "札幌に行って、プログラマーにあいます" or "札幌に行って、プログラマーさんにあいます" to say "I am going to Sapporo, and meeting some programmers?"

Is it used to indicate that you're talking about people who have a certain profession, not the profession itself? Or is it used merely to be polite? I assume using さん after a gairaigo word like プログラマー isn't an issue.