My sense is that wood takes a significant amount of time to absorb or lose moisture. (Bow hair is another matter.)
If the goal is to keep the absolute moisture level in the wood constant (hard to do as temperature changes), as the temperature goes UP, the relative humidity has to go DOWN because warm air has a higher capacity to hold moisture.
But I find that within normal ranges of temperature and humidity during the festival season in the northeast US, I don't worry about it. I go camping with my nicest fiddle and my Martin guitar, play them under cover in the rain, keep it out of direct sunlight for the most part, I put in the case overnight. Other than that, they are out and laying about a good part of the day.
The most damage, I have found, is when I leave a guitar out all winter on the guitar stand when the humidity is 30% or lower for months on end. I have had one guitar top crack by getting too dry and another got a hump in the neck. Both guitars were repaired by re-humidifying first. I never detected a problem with my fiddles in the winter.....yet. And I leave them out on top of the piano all the time, summer and winter.