Add the following PHP code to your child theme functions.php file.

//default checkout US state dropdown blank instead of the default site state making taxes sometimes show up 
add_filter( 'default_checkout_billing_state', 'change_default_checkout_state' );
add_filter( 'default_checkout_shipping_state', 'change_default_checkout_state' );
function change_default_checkout_state() {
    return ''; //set state code if you want to set it otherwise leave it blank.
}