Been doing that for yrs and haven't hurt the edging yet. I just rotate the trimmer head so it's vertical and cut a groove in the grass and dirt alongside the edging. If the ground is rocky you'll,of course, go thru a lot of string.
Mike

Same here....I use the trimmer on it's side as well to go around the black edging & there is no damage. You'll get better with the trimmer as you do it more often but watch out for the hot muffler when you've got it tilted or you can get burnt, literally....

I have cut an English edge (clean straight edge of soil between my lawn and bed) and filled it with mulch. I use a string trimmer. And I keep "kicking up" mulch with it. Did I make a mistake by filling my edge with mulch? Please help!