Mine has been off the charger since 7 am and I have 58% battery left. There is no reason you need to turn it on and off unless you don't know if you will be near a power source some hours later. The Xoom is really easy on the battery.
I don't see the harm in keeping it on all the time. I can't really think of any major consumer electronics I own (besides my TV) that aren't kept on for extended periods of time, so I suspect the Xoom won't be any different.