I file both sides.
Imagine a rectangle (say 4" side to side, 1" top to bottom) with it sliding into the machine with the left end first and then sliding to the left to close. I would file it so that the right side is a hair (or less) wider. That way as it is just about to "seat" it gets tighter.
FYI - A much easier method for tightening a factory made but now loose slide was shown to me by Cathy. She simply gives the plate a slight bend across the short direction and Viola! it is tight again.