If you would like to have “Sign up for our newsletter” box checked automatically for new account sign-up, find the below code:
<input type="checkbox" name="newsletter" id="newsletter" value="1" {if isset($smarty.post.newsletter) AND $smarty.post.newsletter == 1} checked="checked"{/if} />
in the following files of your theme folder: authentication.tpl, identity.tpl, order-opc-new-account.tpl
Replace the above code with the code below:
<input type="checkbox" name="newsletter" id="newsletter" value="1" checked="checked" />
And you’re done with it