I have a 645 and use either the rubber hood or, with my 150mm, the built in hood. I agree that they are really worth it. One thing that is more subtile, is that very slight flare may not show up so much as flare, but a general reduction of contrast.
Check out KEH, they have rubber hoods for next to nothing and bellows hoods for as little as $30-$50. Here is their RB hood page.