The Tor Project releases a new browser bundle about once a month. It's usually a good idea to update, because they include new versions of Firefox, OpenSSL, and Tor, which provide security fixes at the least, and sometimes new features. The latest browser bundle update includes a security fix to Firefox which patches a vulnerability where the Turkish government was hijacking HTTPS connections. There are three exit nodes in Turkey, so if you browse non-onion sites over Tor, you should update.