when charging my 2nd generation shuffle do i keep its power switch on?

It doesn't matter.

I tend to always keep the power switch in the ON position, with my 2nd gen shuffle. When using the shuffle, it "goes to sleep" if inactive for a few minutes. It takes an initial press of any button to wake it up, before it responses to another press to make it play a song. In my experience, I can't tell the difference in battery life per change, whether I just keep the power switch in the ON position all the time or constantly flip it between OFF and ON. So, to avoid wearing out the power switch (and also avoid dealing with a tiny switch all the time), I just keep it ON most of the time.

Considering how old the 2nd gen shuffle is at this point, I'm a bit amazed that the battery in mine still holds a good charge. I also have a 1st gen shuffle, and its battery has become worn out (no longer holds a charge for more than about 20 minutes after disconnecting).