Grant,
<font color=blue>"...they didn't do a very good job of grading over the laterals"</font color=blue>
Do you mean the area over the septic tank, the area over the leach field, or area over drywell? In any of these cases, there should be no problem in lightly grading over for reseeding. All these septic components are buried at least 1-2 feet (or they should be if properly installed) below the surface and I don't see what harm you can do by boxblading and reseeding. Hope this helps.
Regards,
Bob Ancar
Cambridge, NY