Does everybody let content creators know when they're using their products in an image? 

I've found that they mostly respond to it. With many Poser artists having thousands of free and paid items in their inventory, most Poser products will never see much use. When one finally does get used in an image, it's fairly special.

There have occasionally been posts by content creators wondering if their products have been used.

I've usually gotten good responses when I let people know I'm using their products. They'll often drop a comment on the gallery after they see it.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon