Intersting point. I had not considered that, really, tho I did take care to try and keep the shunt resistors cool.
Actually, for the Seasonic PSU, the ambient was 21C and the max seen in the box was 46C, so the temp rise was 25C. I don't think it has ever gone higher. Higher power PSUs have more airflow, while lower airflow PSUs don't put out as much heat.
3 of the shunt resistors are bolted tightly to underside of the front panel of the DBS-2100 PSU load tester. In other words, the top panel, which is made of fairly sturdy steel, is being used as a heatsink. On the right side where they are located, there are no load resistors directly beneath them. One of them is actually bolted right up against the front edge vent intake of the DBS-2100. The grill is visible in this photo:
This grill is the only way intake air gets to the 4 fans blowing out on the other side. I've described in the previous PSU test rig article that these are mid-speed 80mm fans undervolted to 5V; I'd guess they each push maybe 10-12 cfm in free air; inside this tightly packed PSU loader, it's probably half of that -- maybe 20~25 cfm in total. All three shunt resistors are quite close to the air intake path, and you can feel a fair amount of air being pulled in at these vents. In other words, even though the shunt resistors are inside, I doubt we'd see 30C above ambient in their location.
As for the 4th shunt resistor, it is bolted to the aluminum inset panel for the 5 switches for the +12V2 load bank. The screws for this resistor can be see in this photo (the 2 in the center):
Because of the heatsink action of this aluminuim panel, one side of which is on the outside of the thermal simulation box, the temp of this resistor is considerably lower than that of the internal PSU "intake temp" sensor that is positioned in the box about 3 inches behind it. When you put your fingers on that panel, even when the PSU is at >400W load, it never feels more than warm.
So I suspect inaccuracy due to shunt resistor temp rise is not really a cause for concern, espcailly as the potential error is so tiny compared to potential errors elsewhere...
But thanks for bringing my attn to this.