If I'm not mistaken, most of the options are set by radio buttons... that means that when you sign up, those are set by default to one of the options.
If you want to change the default options, simply edit the templates registeradult and registercoppa.
There you will see something like this:
PHP Code:
<input type="radio" name="allowmail" value="yes" checked> yes
<input type="radio" name="allowmail" value="no"> no
If you want to invert it, put in
PHP Code:
<input type="radio" name="allowmail" value="yes"> yes
<input type="radio" name="allowmail" value="no" checked> no
(Move 'checked' from one option to another)