No, you can keep the iPod charging most of the day without an issue of ruining the battery. It's not bad to do this because the iPod (and iPhone) use a lithium-ion battery, which use a trickle charge with a sensor to monitor the battery and keep it charged at its peak. Once the battery charge hits 100%, it will stop charging. Also when using a plugged in iPod (or iPhone), it will use direct power and not the battery.
- Lithium batteries do need to be used to maintain their best health. So best usage is frequent and light use (with few full dischages).
- Charge the battery often. The battery lasts longer with partial rather than full discharges.
- Avoid full discharge because it wears down the battery. 80% depth-of-discharge recommended.
So, it's actually best to keep the newer iPod's and iPhone's plugged and charging most of the time.