this is the file for registration page
Themes/default/Register.template.php
to make "allow users to email me" checked
find
<dt><strong><label for="allow_email">', $txt['allow_user_email'], ':</label></strong></dt>
<dd>
<input type="checkbox" name="allow_email" id="allow_email" tabindex="', $context['tabindex']++, '" class="input_check" />
</dd>
in that file, change to
<dt><strong><label for="allow_email">', $txt['allow_user_email'], ':</label></strong></dt>
<dd>
<input type="checkbox" name="allow_email" id="allow_email" tabindex="', $context['tabindex']++, '" class="input_check" checked="checked" />
</dd>