Personally, I've opted to keep these local. It's true that there are a few older films where "costumer" is the only costume-related credit, but I sure wouldn't want to track any kind of "costumers" on modern-day films, and that's exactly what would happen when "costumer" would be added to the crew credits table. While I'm all for tracking the right person despite the exact job label, I'm terrified that someone comes along (and someone's bound to) that thinks: well, if "costumer" is okay here, I'll enter them every time. That, and that alone, has kept me from submitting these. A blanket "no costumers of any kind" seems to work better than a conditional ("only if none of the valid costume designer job titles appears in the credits, then enter the "key costumer").

So I only track them when none of the valid job titles from the rules appears in the credits, and even then, I keep them local.