Feynman/Einstein/etc and Basic Maths Did famous physicists/mathematicians do basic arithmetic (add/sub/div/mult) in their heads, or would they work them out on paper?

Is there a scale of difficulty for this?  I.e. "Erdos could do 5 digit multiplication in his head, but Feynman could at most do 3 digit reliably".

Or do they all do it on pencil/paper?
It to a large extent depends on how much logically one thinks. One can do the same by breaking a big problem into smaller simpler ones. In Feynman's 'Surely you are joking Mr. Feynman',  Feynman mentions how he defeats a Japanese abacus expert.. simply by breaking a problem  logically into known steps.
Does anyone do all basic arithmetic in their head?  No, so the answer to your first question is that they certainly did some basic arithmetic on paper.

As far as a scale of skill in mental math... I would sincerely doubt this exists.  This would have to be similar to a World Records maintaining group, and maintain a record for all "famous" researchers.  Of course, then they'd have to define famous.

I would be willing to bet that you will only find individual stories of the mental math capabilities of mathematicians or physicists, but with no organization maintaining a ranking system or scale.

Related: Euler carried a math problem to 50 decimal places in his head: http://library.thinkquest.org/22494/stories/Euler.htm  Then again, he was Euler. :)