Basically yes, you will not get a correction notice to change anything if the temp is below 78 and you did not make adjustments because the load will still be less than the adjusted ampacity of the conductor, but not making adjustments the other way results in the load being more than the adjusted ampacity of conductors, if you don't pay attention to this requirement.
I would also look at the exception to 310.15(A)(2). It gets you out of needing to adjust in some cases if the higher temp portion is a limited length and the rest of the circuit is in a lower ambient temp. Good example of where this exception is used a lot (and maybe some don't even know it but get by anyway because of the exception) is when supplying rooftop HVAC units - or anything else on the roof for that matter. If you poke through the roof right near the unit you likely can use the exception, if you run across on the exterior of the roof - you very likely will need to adjust the conductor ampacity.
