iPhone consistantly plugged in: is this a bad idea?

I want to use my old iPhone 3G S as a proprietary audio output device. I put my entire iTunes library on it and a couple of apps like Spotify. I listen to music maybe 3, 4 times a week, but when I do I like the flexibility the device gives me over using CDs or plugging in my new iPhone, which I use for messaging and all sorts of other stuff. In order to do this I have my old iPhone connected to a charger, which is always on, and plugged into my amplifier.

My question is: is this a bad idea? Should I unplug the charger when I'm not using the iPhone, or can I just leave it plugged in? I'm not too worried about decreasing battery life; this iPhone is not going with me on any long trip. I'm more worried about power consumption, or the possibility that I might blow it up if I don't unplug every month or so. Anyone have any experience with this or using a similar set-up?