Computer Question: Turn it off or leave it on?

Ok, what is the current wisdom regarding turning off your computer every day when you are done using it? They say in order to keep your electricity bill lower, turn it off...but there are those who say by turning it off every day, you wear out the hard-drive quicker from having to re-boot every day. But I know that some conventional wisdom of the past has changed...like warming up your car. With the advent of electronic ignition and fuel injection, it really isn't necessary to warm up your car in cold weather. So, which is it with computers, turn it off or leave it on?