How important is the temperature stability for gain - setting resistors?
You cannot assume the temperature drift of two resistors next to each other on a reel is similar unless the data sheet, or other documents explicitly state this.

Read the data sheet is my advice.  You can buy resistors as a pair whose ratiometric temperature stability is intended to be very small ie if one drifts a little bit high, so will the other one thus ensuring gain is stable over temperature.