My Mac laptop is never turned off. When I am not using it, the lid is closed and it is in "sleep" mode until I open it again. It is always plugged in. I only turn it off (or use battery power alone) when I am traveling or transporting it. That said, the laptop is my secondary computer; I use it mostly in bed and do most of my computing on a desktop machine.
I have never had a problem with my current Macbook or my old Powerbook using it this way.