[sdiy] Newbie questions about summing buffers

[ganesha]

Hi,

I have 2 questions about summing buffers
If you take this schematic:

http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Op-Amp_Summing_Amplifier.svg/200px-Op-Amp_Summing_Amplifier.svg.png


-Most of the schematics I've seen so far use large resistor values of f.e. 100K for resistors R1- Rn and Rf. 
Is there a specific reason to use 100K 0,1% instead of  small resistor values like for example 1K 0,1%? If they both are high precision resistors of 0,1% I would think the result would be exactly the same?

-In some schematics I've seen a small capacitor in parallel with resistor Rf. Can someone explain when it is better to add a capacitor?

Thanks.