I use 2 times the OD of the ferrule as a good rule of thumb for a minimum amount to wrap. Different manufacturers suggest different amounts, so if you have a warranted blank, make sure you meet the minimum required so you don't void the warranty. There is even one brand out today that doesn't require any.
I have never wrapped the entire ferrule area. It looks bad (IMO) and adds a fair amount of weight by the time you finish that entire area. That would probably screw up your action, especially on a multi-piece blank.

[This message has been edited by ol' blue (edited 12 January 2006).]