So, how valid is this?
"A hydrometer reads accurately at 60 Fahrenheit. Temperatures above and below 60F require that you add or subtract from your reading."
Yes, there is a temperature correction for hydrometers. Check out this site for an explanation on hydrometers and a table for the temperature correction. http://www.howtobrew.com/appendices/appendixA.html
I like howtobrew.com in general, lots of good information.