I tested my vents when I got to work this morning. Ambient temperature when I left home was 18F. I have a 25 minute commute. My coolant temperature was 190F when I started the test, and 177F when I ended the test. The test took only a few minutes. I used the multimeter I've shown previously. It has a k-type thermocouple to measure temperature. Once I got to work, I put it in neutral, put the heat to 89 and fan to max. I inserted the temp probe in the far passenger side and the temperature got up to 137F. I then moved it to the driver's side and it got up to 137F. Granted, the coolant temperature was dropping. So, if I could measure both, I would have to say that the driver's side is probably 1-2F, and maybe 3F at the maximum. I then tested it again. I moved the temperature probe to the passenger side again and it went up to 135F. Then, again to the driver's side and got 134-135F.
I'd say if you have more than 10F variation, you definitely have an issue. A small issue at that point, but an issue.