~~~~), both for the article and non-article namespaces, is good practice, and facilitates discussion by identifying the author of a particular comment. Other users can then navigate to a talk page and address their comments to the specific, relevant user(s). Discussion is an important part of collaborative editing, because it helps all users to understand the progress and evolution of a work.
Comments posted on user talk pages, article talk pages and other discussion pages should be properly signed. Signature use that is intentionally and persistently disruptive may lead to blocks.
When editing a page, main namespace articles should not be signed, because the article is a shared work, based on the contributions of many people, and one editor should not be singled out above others.
Signatures on Wikipedia identify you as a user and your contributions to Wikipedia. They encourage civility in discussions by identifying the author of a particular comment and the date and time at which it was made. Because of that, having an uncivil signature is strongly discouraged (in some cases, to the point of blocking the user until it is changed). In general, anything that is not allowed in a username should not be used in a signature either.
Furthermore, signatures also serve a technical purpose: various user scripts and talk-page archiving bots, including lowercase sigmabot III, rely on their time stamps to know when to archive old threads. It's because of this that it's also important to avoid overly customizing the date output of a signature, as doing so can lead to stale threads persisting long after they'd otherwise be archived.
Any posts made to the user talk pages, article talk pages and any other discussion pages must be signed (this does not include adding WikiProject boxes and other such administrative templates). Note that, as stated below, if you choose to edit those kinds of pages without logging in, then you should still sign your posts -- regardless of whether or not you have an account. Edits to articles must not be signed, as signatures on Wikipedia are not intended to indicate ownership or authorship of any article. Instead, the page history identifies who made edits. Signatures do not translate from
~~~~ in edit summaries. In other instances when posts should not be signed, specific instructions are provided to contributors. Files used in articles, including image and sound files, should not contain any Wikipedians' signatures.