I’m sure there are, and you could search the ‘net... I run iStat menus myself, and assume(d) that the default parameters are either read from the Mac, or the developer knows what they are for all the macs that they support.
Temperature readings are simply just that - read from the sensors that have communication features.
I find it useful to have this app, and I keep my temp readings in the uppermost Finder bar. FWIW I only monitor 2 sensors on my Mac Pro: the Northbridge heatsink and the diode (running at 48 and 57°C) as these are problematic in the 2008-2010 models. (Heatsink separation due to broken nylon connectors is the issue.)

I had just replaced the RAM sticks with 16MB that have heat sinks, and my temperatures ramped up 15° (to about 70°C) to the point where my Mac would shut down without warning... so I used the custom settings feature of iStat to increase various fan speeds; now temperatures are actually further down than they had been before the RAM upgrade... Yes, I did vacuum out all dust I could see...

...so for anyone with high temps, at least it‘s a stop-gap method to increase fan circulation if you can’t get around to de-dusting the computer just yet... and it does let you know all kinds of other stats too...

Cheers
JR

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Are there established norms and “uh oh” values for these parameters? My assumption is that if these are fine, it’s unlikely that dust collecting inside the machine is not the issue.

Thanks for the suggestion.

Jim Robertson