Copyright and Collage

In response to my sister's mention of investigating copyright rules related to using materials for collage (magazine adverts, I'm thinking wrapping paper), I revisited the U.S. government website that has all of the rules and regulations.  There are an enormous number of articles and publications on the subject, most of which point out that the rules and regulations are an unclear and slippery slope.  What do collage artists do with respect to others' work?  Use so little of it or use it in such small batches that the original artist won't notice?  Figure that their work won't be noticed in terms of using someone else's work while still undiscovered in the art world?

Any thoughts?