When I had a small lawn and would sharpen my blade with a hand file (didn’t have anything fancy to work with) I would just get it to a sharp edge again and could tell the difference between a cut and hacking. A straight edge isn’t that important, just getting rid of the rounded surface. And balance as mentioned above. When the pits and dings begin to affect that, then get a new blade (or try to balance it out, but that’s a bit more work).