There is still on going argument's about that, but I can tell you
from my experience that,
I
have never had a Computer to die on me from turning it off.
The rule is, If you are going to use your Computer in a couple of
hours again leave it on.
If you are going to bed put your Computer to bed too.
However at night would be a good time to defrag your harddrive, or
download a large file,
unless it's in your bedroom. and the least bit of noise
wakes you up.
Who wants to wake up to you got Spam every ten
minutes.