I agree with you there. There shouldn't be any worry about turning the hard drive on/off. I have a hard drive that worked for 5 years, another for 3 years so far, and they go to the on/off cycle at least 5 times a day (they're still working). I also have a computer that I turn on/off at 2 or 3 times a day that's 5 years old, and it's still working though a replaced it a few months ago.
If you're worry about too much stress on the computer, I think you should at least turn off the computer when you go to sleep. No reason to waste energy.