When editing posts for grammar and spelling, try to keep edits light handed. A few simple rules I try to use:
If you are editing a post because it is incorrect (Starp -> Strap) then it's a good cleanup edit.
If you are editing a post because it's unclear, then it's also a good edit.
If you are changing something that is clear and correct, but not the way you would say it... that's a bad edit. You are replacing the authors voice with your own.