There's some interesting info in this 2003/2004 Study Guide for becoming a USRSA Certified Stringer http://www.racquettech.com/studyguide2003.html In the Customer Service section there are sub-sections on String Selection String Technology and a Back to Basics section on stringbed stiffness (or tension)
Buying a Stringing Machine I'm sure I posted this document ages ago, but I couldn't find it. Some advice for people considering buying a stringing machine.