Even when your PC is switched off the power pack still takes power so when you push the 'on' button on the front it starts up, so the best bet is to unplug it from the wall to save the most power....
A peripheral consideration here is that most hardware failures occur at power-on/off. Modern disk drives will spin happily day-in day-out for years. I run my home pc all day even if I am out. My work pc is never shut down because I frequently need access from home. It has been fine, apart from planned re-boots, for 2 years.