You render when you want to see the best possible preview of the Timeline content as played by in the Edit Mode monitor.
It is your opportunity to catch a problem sooner than later.
It is especially important previewing of effects, transitions, and non native formats.
You probably know that already, but...
Generally speaking and depending on the project, I would probably opt to determine what I have during the project and build on that, rather the wait until I think that I have finished all and was ready to move on to disc menus - Publish+Share choices.
As ATR points out, Rendering is just for getting the smoothest playback.
I do many Projects, that never get Rendered, prior to output. However, those are Projects where the Source Footage matches the Project's Preset 100%, and do not have many intricate Effects in them.
OTOH, I have some, where I AM using intricate Effects, and I might end up doing a Render of that portion of my Timeline many, many times, until I am 100% satisfied. I limit the area to be Rendered with the WAB (Work Area Bar), so that I am not Rendering any other parts of my Timeline.
This article goes into just a bit more detail on using the WAB to limit the area to be Rendered: http://forums.adobe.com/thread/794719 and there is a link to a WAB article, near the end.