The default wikipedia signature field, in preferences, is empty, and the "signature contains wiki markup" checkbox is unchecked. This combination results in a simple, valid, signature, containing a link to the user's homepage (hyperlinked from their username), and the date.
If the "contains markup" checkbox is checked, and the signature text field left empty, the generated signature contains the same simple text, but the username is no longer a link to the user's home page. Using such a signature will result in scoldings, and comment re-signing, by the sinebot.
If the text field is empty, then the supplied default signature should be valid no matter what. That is, the state of the checkbox should be irrelevant.