BAL is absolutely right, the maximum power available from your PC for the WHOLE USB chain is 500mA per USB port.
My USB scanner takes up to 500 mA when in use so if I had an unpowered hub with my scanner, then connected some other bits and pieces, I would expect some strange behaviour if I used them all at the same time.
You're probably OK if you've only got a printer connected as that won't take power from the USB, only its AC input. Once you start adding things like scanners that draw all their power from the USB supply, you'll need to power up the hub to ensure reliable operation.
You are unlikely to damage anything by unplugging the power to the USB hub while it's on, but some devices (like digital cameras) really hate you doing that when they're transferring data. Put it this way - it's not highly recommended!
Hope this helps.