When tuning the primaries for a Qjet (may apply to all carbs but I'm just concentrating on Qjet since that's what I have), do you always change jets AND metering rods at the same time, or can you just change jets and leave the same rods, or vice versa?

Go to Edelbrocks web site and look at the carb tuning manual for the performer carb. This will give you an idea of what you can do with a q-jet.

with Q-jets all primary metering rods have the same diameter power tip. So tune for wide open first (changing primary jets, secondary rods)then increase primary meter rod area to lean dowm to a satisfactory cruise mixture. Becarful not to mix newer and older primary metering rods because they are different lengths. I think the cut off year was 1974. All q jets with number that starts with 170xxxxx are newer all q jets and number that starts with 70xxxx are older.

In short you dont have to change both primary rods/jets for just part throttle. For just part throttle I would just change the primary metering rods
also you can adjust where the primary metering rods stop in the bottom with a screw thats behind the plug in the center of the base plate in front (older) and below a plug in the carb top just in front of the carb vent tube.

The secondarys jets are fixed orrifices so you change mixture with metering rods alone
If you plan to mess with Q jets a very helpful book is Rochester Carburetors by Doug Roe
hope this helps