Actually, fast temperaturechanges are the worst kind of stress for any electronic or electromechanical device. Every time you turn on your computer, after it has been cooled down, you're putting another nail into it's coffin, figuratively spoken.

Waking up your harddisk from energysafing mode is almost as bad. I lost 3 drives to this nonsense within 2 months. That was back in my irc times, before emule.

Running always, but on a nearly constant temperature is the best way to go. Provided you keep the temperature low by appropriate cooling measures.
The increasesd energy bill is by far outweight by saving money from NOT having to buy new stuff.

The second best treastment of your harddisks is to NOT use them at all. But who want to do that ?