The short answer is "no", you won't damage the 'book by leaving it on all the time.
Keep in mind that the MacBook can run very warm at times. I would not leave your laptop unattended when it is actually running. Many people that I know, simply shut the lid, and the 'book goes to sleep immediately. Some people go for weeks or months without ever shutting the 'Book off. Your 'Book uses very little power when asleep, and wakes up within a couple of seconds simply by opening the lid. If this is what you mean by leaving it on all the time, then it's very safe to do that, and a good way to use your 'Book.
If on the other hand, you mean powered on, and you leave the lid open, and you have it set to never go to sleep - then, I would recommend not doing that, unless you monitor the temperature.
I have had a couple of MacBooks that will not sleep when closing the lid. They had hardware problems, and were repaired under warranty. Always watch to see that the 'Book actually goes to sleep when you close the lid. (the LED next to the IR port will start flashing after a few seconds, and you can see that the Apple on the top of the lid goes dark.