Bear in mind the Sign Up checkbox only appears on checkout pages we manage for you, namely our Free Checkout (for 0.00 order totals) and card-based direct payment checkouts using Authorize.Net or PayPal Payments Pro.
Checkouts paid through PayPal, Google Checkout, ClickBank, 2CheckOut, or TrialPay would occur on those payment processors' respective checkout sites, so we have no control over them, but PayPal checkouts in particular do not have any opt-in/-out checkbox, so all buyers who pay by PayPal would be opted-in automatically.