Any time you see a post that needs improvement and are inclined to suggest an edit, you are welcome to do so.1
However, edits should not be an end in itself but should be a value-adding activity. The aim of edits is to increase the probability of enhancing the satisfaction of readers.
Edits are expected to be substantial and to leave the post better than you found it.1
Thus, edits should be effective; the improvement should not be negligible.
If the edit makes no improvement whatsoever, it should be rejected.
This edit fails to make the post even a little bit easier to read, easier to find, more accurate or more accessible. Changes are either completely superfluous or actively harm readability.2
On German Language Stack Exchange, any correction of the German text is considered a relevant improvement and is therefore a justified edit. The term correction is generally defined as ‘action to eliminate a detected nonconformity’. Here, nonconformities may relate to orthography, grammar, punctuation, or typography.
Edits are not limited to corrections. Posts may be improved by additions or by changes of the wording. However, such improvements can affect the message and intentions of the post. In this respect, the requirements for editors are similar to those for translators: Editors should provide corrected or otherwise improved versions that accurately preserve the message and intentions of the original post. Faithful edits retain both the intention of the original author and the style and register of the language used.
Edits to questions should not affect the choice of the right answer. In particular, edits to questions should not invalidate any already given answer. (This also applies to the original author of the question. If necessary, a new question may be asked.) Edits to answers should not affect the conclusions from the answer.
If the edit deviates from the original intent of the post, the original author may want to consider rejecting the edit. Edits that clearly conflict with the author’s intent should be rejected.
This edit deviates from the original intent of the post. Even edits that must make drastic changes should strive to preserve the goals of the post’s owner.2
Nevertheless, mere corrections as defined above should not be rejected. (This also applies to the original author of the post.)
If you are not comfortable with the idea of your contributions being collaboratively edited by other trusted users, this may not be the site for you.1