Well, data center operations is a pretty competitive business with many acquisitions happening. Especially larger companies, like Equinix are buying up smaller companies. The large ones have entire teams dedicated to energy efficiency, both for cost savings and due to regulations popping up.
Coincidentally I've worked with a guy that does exactly this. His entire job is figuring out ways to make the data centers more efficient and I know that there are working groups where different operators come together to discuss ways to save energy.
So I'm sure not that many people will know the optimal temperature, but the ones that need to know, know. Small, on premise data centers might be winging it, but the ones that are running the majority of the internet are really trying to improve efficiency, just to stay competitive.