If you have themes on your site, that you are not using, delete them. You might have uploaded them to your site to try them out and see what they look like, but decided against using them. Do not leave them sit there. If these themes are not up dated they could be a security risk to your site. The best thing to do is delete them.
But remember always keep an extra theme on your site, in case your main theme crashes The one I keep is WordPress own theme, Twenty Twelve (or their newest theme) Even thought I do not use the Twenty Twelve theme, I will always keep it up to date.