Canon S9000 Heats Ink?

I have been told that ink jet printers heat the ink sets and so it is best to turn them off when not using. Supposedly it will cause some evaporation of the inks. Is this true? I have been leaving my S9000 on all the time and have not had a problem in 9 months. However, I can't say if the inks are evaporating. I don't feel any heat when checking the ink cartidges. I did a search and could not find any reference to heat and inks.

John