When I am making a project with several fabrics I stack the with the folded sides aligned. I trim the selvage side so that all my fabrics are the same width. I start at the end and slice the selvage about 18" or so. I don't cut it off at the point tho, because if I use all the fabric, I will let the salvage hang on until I get to the end. Then I have long strips of selvage that I can fold neatly and store in a drawer.
If I'm trimming one piece of yardage I determine what width of fabric I need then trip the leftover on the selvage side. Sometime I don't even get a usable width and toss it, but usually there is a couple of inches.