Won't hurt the battery at all, if it is fully charged then it will stop charging. You just need to do a full charging cycle once per month to exercise the battery.
And just to add my 10 pence worth, you most definitely do want it to charge whilst it's connected to your computer because doing anything on your iPod (like syncing it with iTunes for example) is a huge battery drainer and if the iPod were not receiving a charge whilst doing this the battery would be empty within minutes (if not sooner).