No problems
If it's a fifth-gen iPod, it's battery is probably weared down quite a bit already.
It shouldn't make any difference. Are you seeing any problems or is this just precautionary? I have my iPad technically plugging in (charging via USB from my laptop) all the time and I never get any trouble. I don't track when it reaches 100%.
It trickle charges after it fully charges. You could leave it plugged in 24/7 and it would be fine.