It's the same with your lawn's edging. Do not "scalp" the edges of your lawn while trimming. This always leads to an infestation of crabgrass and other unsightly weeds along the edges. Use an edger that has a blade that cuts straight up and down, using the "slice and edge" approach along the borders of your lawn. If you have a string trimmer and would like to use it for edging, turn the cutting end 90 degrees from the surface being cut to slice the edge.
Remove a "narrow slice" along the edge of your lawn, rather than opening up a large gouge of soil when you do your edging. Exposing the soil along the edging only establishes a perfect seedbed for crabgrass and other ugly weeds to take hold, ruining the beauty of your lawn.