BTW- I fully realize that the words "100mV of output offset error" are likely to cause aneurysms among some people who design op amp circuits. Pete's right that driving the op amp from a lower impedance input is the best of all possible solutions.. you get good output and low error all at once.
Sometimes your input signal isn't strong enough to drive a lower input impedance though. Then you have to consider the tradeoffs.
.. and I do still think his estimate for Rin is kinda low.