It would be great if this topic could be definitively resolved but the truth is there is not a "one size fits all answer" to rsherid's question.
What I can say is that in a general sense the industry says; "if you have an Intel processor (like my MBPr) a CPU Core temperature of 40-45 degrees Celsius while idling and/or a temperature of over 80-85 degrees Celsius under full load
may be cause for concern".
But this is a general guideline and there are differences between CPU temp tolerances of 5 degrees Celsius just between different processors in the same model MBP.
Anecdotally I have a 2015 MBP with MFC installed and set on Auto. The fan speed range is between 1299-6199 RPM, at 45 degrees Celsius (idling) the fan does not come on at all (Menu bar icon shows zero RPM).
From my observation the Menu Bar display is based on CPU Proximity rather than one core or another (on Auto).
The temp goes up and down between 45-50 degrees Celsius without initiating the fan at all but if the temperature is
sustained the fan comes on.
If I am running a video converter or a CPU intensive game it is not unusual for the temp to reach 80-85 degrees Celsius and stay there but the fan still does not reach maximum rpm.
I have been using MFC for over 5 years on three different Mac books so I'm fairly confident that it works and I have never had a Mac Book fail on me at all other than hard drive failure due to age and/or data corruption or in one case a keyboard failure on a new device leading to device replacement and that is the device I'm using now.
So I can say fairly confidently that a 2.9 GHz Intel Core i5 CPU running in a MacBook Pro (Retina, 13-inch, Early 2015) at ambient temperatures of about 25-30 degrees Celsius (under a ceiling fan) is happy to run at temperatures of 45-85 degrees Celsius and higher (for short intervals).
I have only a few times seen the fan speed approach 5000 rpm and it is quite audible at that point and quickly drops as does the temp.
Its likely that the idle temp would be lower if the ambient temp was too but I suspect it would soon rise to about 45 deg on use.
Always remember that Mac's have built in protection that will shut the device down if it gets too hot.
N.B. It's important to note that MFC fan speeds are different for different devices, models and processors, eg AMD processors generally have tolerances around 5 degrees Celsius lower than Intel.