I've repaired a few multimeters recently and needed something to check the resistance ranges. I wanted a reference with values between 10Ω and 1MΩ with 0.01% tolerance, which should cover most multimeter ranges.
All the resistance standards I've seen use separate terminals for each resistor. I'm lazy and didn't want to move cables around to select resistors, so I used a rotary switch instead. Using a switch also ended up costing a bit less - gold-plated Pomona banana jacks aren't cheap!
The switch introduces some extra resistance, but from my measurements, it's only around 10mΩ, which is fairly negligible. I'll be using 4-wire sensing most of the time, which should eliminate switch resistance.
Are there any obvious downsides anyone can see with the design/construction? My main worry was mechanical stress on the switch influencing the resistors. I mounted the resistors a couple of millimetres off the PCB to give them some mechanical isolation. So far I haven't been able to measure any difference in resistance when applying force to the switch - at least with a 6.5 digit meter. I'm working on an SMD version, so perhaps it would be more of an issue there.