Joomla! Documentation™

In most cases, probably not. Joomla 2.5 will continue be supported until December 31st of 2014 and you can update directly to Joomla 3 once it’s tried-and-tested thoroughly by other users. You can even wait until Joomla 3.5 with release scheduled for March 2014 and still get a direct upgrade. The only reason you should update is if you need Joomla 3’s features or want to be on the leading edge.