I always pre-wash my fabrics before they enter The Stash, and sometimes I notice that the selvage will shrink more than the rest of the fabric. (This is one of the reasons to not use selvages in your patchwork!) Even after pressing, I end up with a gently curved edge along the selvage, which I don't want. I want a nice, straight edge to start with so all my long cuts parallel to the selvage are also nice and straight.
Now, if I just plunk my rotary ruler down and trim off that wonky curved edge as it is, I will end up with a cut edge that is curved in the opposite direction. Still not what I want.
To fix this problem, there is a simple solution. Snip into the selvage every few inches with scissors, for the whole length of the fabric edge. This will relax the shrunken edge and, after you re-press it, your selvage edge will lay straight.
Now you can trim off the selvage and be sure that your freshly cut edge is straight, too.