Create them within CB > Field Management and set them to display on registration. Next right click them in Chrome or Firefox with Firebug and click Inspect Element. Next you'll see a div with cb_form_line class higher up the DOM from the input. That div will have a unique ID. You can now use this to specifically hide a field row using CSS. Example as follows.
Code:
#cbfr_42 {
display: none;
}