I have not been able to find definitive guides per se, although some books reference optimal tempeature for computers.
I found some articles that refernce the exact tempeature of 68 degrees.
http://zdnet.com.com/2100-11-257567.html
http://www.innopacusers.org/list/archives/2002/msg01328.html
BTW- I was just being a smart ass before.
This really boils down to common sense, kind of like, you get up and brush your teeth in the morning or put on deoderant.
There is really no "guide" that tells you to do it. You just know that it needs to be done because common sense and "passed down" knowledge tells you to do so.