This is my thoughts as well. I don't see a need to jump through extra hoops to default the signature to an off state for users that want to use signatures. The reason why its my signature is I want to set it and forget it, not need to remember to check the box every time I post it.
As mentioned, you can disable the view of signatures by changing the setting in your user control panel > Edit Options section.
I do like the idea of perhaps having it unchecked on each subsequent post. That would require a code modification for though, so I doubt we'll see that happening.
As of PHP 5.5, the MySQL library has been officially deprecated. It is recommended to move to either MySQLi or PDO libraries for your mysql connectivity. See here for help choosing which interface you prefer: http://php.net/manual/en/mysqlinfo.api.choosing.php