If your edit improves the quality of the post in even a small way, feel free to edit. Sometimes it's something as little as changing "i" to "I" or adding a question mark, but it should still be done in my opinion, if it indeed improves the quality of the post.
Remember, people from all walks of life read the questions on this site eventually, and almost every question/answer will be of some benefit to more than one person at some time. Therefore, it's best to have questions and answers at the highest possible quality, because quality makes improves readability, and to besides, it lends some credence to a question or answer if it "looks good" ;).
Personally, I have no qualms accepting such minor edits when reviewing suggested edits, because in fact I encourage them (the minor edits), and anything that in general improves the quality of the site and its content overall.