They need to be high accuracy types, carbon film is useless here, 0.5% or better metal film is good as they have accuracy and low tempcos, and ideally they should be thermally tracking each other (ie close together, away from heat sources). Even 1% change in one resistor will swing the output full scale for a strain-gauge bridge circuit, you want resistors that are stable at ppm levels (to match the stability of the strain guage elements).