Any edit that improves the post is helpful. This not only improve the overall quality of the site but also helps the user( with <2k rep) to gain reputation(2).
However, editing the question or even the answer brings the post back to [the main page][1]the main page, if it wasn't. This can bombard the main page with old/relatively-insignificant posts and thus prevent the more relevant questions from grabbing the intended audience's attention.
This makes it important to prioritize whether to improve/approve the edit or reject it.
IMO, if the question is relatively new(a few hours or so), the edit, if constructive, may be harmlessly approved/improved.
However, for older posts, only if the post significantly improves the quality of the post, should it be approved/improved.
Thus, an edit is minor if it does not significantly improve the post; approving/improving it will compromise with the attention newer posts get. [1]: http://askubuntu.com