I've been told that using the power button to turn off/on your computer is called a "forced shut down", or a "forced boot". I don't know if using the power button in this manner is bad for your computer or not, but I do know that I have occasionally had to do a forced shut down for one reason or another and when I go to turn the computer back on I get a screen that says, Windows was not shut down properly what do you want to do?? And then it gives the options of Start Windows normally, Start Windows in Safe Mode, Start Windows in Safe Mode with Networking. I always just choose start Windows normally and it boots up just fine.
So while it may not do any harm to your computer to use the power button to turn it on and off, that screen comes up for some reason when I don't shut down Windows the recommended way which is to go to Start, then shut down, and then just let the computer power itself down. I would imagine that there is a certain order that is used to shut down Windows and when you just press the power button to turn it off you are bypassing this process. So if possible, I use the recommended way to turn my computer off.
I'm sure there is someone on this board that has more detailed knowledge of the process that can give you a more technical answer as to the best way to shut down your machine.