rendered hitting question is not about RichFaces but about JSF in general. You need to be carefull by using EL in rendered because JSF checks the condition at every phase (in order to decide if the component need to be processed on the particular phase, rendered could be changed before any phase by some custom phaseListener).. read this article related to some real applications performance problems. http://jsfcentral.com/articles/speed_up_your_jsf_app_1.html
the only additional thing related to RichFaces Ajax I want to mention that you should not reRender conditionally rendered element. some parent should be reRendered, because ajax could not insert the content that wasn't already in the DOM.. It could only update existent nodes.