When does a view (or layer) require offscreen rendering?

Hello
this weekend I started to watch the 2011 WWDC videos. I've found really interesting topics about iOS. My favorites were about performance and graphics, but I've found two of them apparently in contradiction. Of course there is something that I didn't get. The sessions that I'm talking about are Understanding UIKit Rendering -121 and Polishing your app -105.
Unfortunately sample code from 2011 is still not downloadable, so is pretty hard to have an overall view. In one session they explain that most of times offscreen rendering should be avoided during visualization in scrollview etc. They fix the performance issues in the sample code almost drawing everything inside the -drawRect method. In the other session the performance issue (on a table view) seems to be due to too much code in the -drawRect method of the table's cells.
First is not clear to me when an OffScreen rendering is required by the system, I've seen in the video that some quartz function such as: cornerRadious, shadowOffset, shadowColor requires it, but does exist a general rule?
Second I don't know if I understood well, but it seems that when there is no offscreen rendering adding layers or views is the way to go. I hope someone could bring light about that..
Thanks,
Andrea