The power available to USB is fairly tightly controlled so, no, it is unlikely to have any impact at all even if you have lots of things plugged in to different USB ports. Of course, unless it is one of the "special" USB ports, it will power off when you power off (or put to sleep) the computer.
However, leaving the iPod dangling from the connector is VERY likely to damage at least the cable if not the iPod connector over time. Not a good idea I'm afraid. It could also damage the USB connector on the computer.