agk - you make a very good point - it is important to keep the solution cost effective and avoid over specifying the precision needed. As a rough guide, in a 10 bit ADC a 1LSB resolution equates to about 0.1%, so resistor ratio precision of, say, 0.01% would be wasted. Further guidance on selecting resistive networks for ADC design can be found at http://www.welwyn-tt.com/pdf/application_notes/LADDERNETWORKS.pdf
These are precision resistors. They are expensive and when used raises the cost of the product very much. The gain fixing resistors,biasing resistors, filters,attenuators,voltage reference circuits,basic arithmatic operations using resistors all need precision resistors. The level of accuracy required depends on the over all accuracy expected in the finished product. A simple estimation can be made from the ADC's no of bits used to process the analog signals.