I have quite a few ipads and most of them are plugged in most of the time. It's generally better for the battery. Not so much the fact of being plugged in and not running out of the battery, but because it avoids deep discharges (under 20-30%), which is what deteriorates battery cells more. I only unplug them when I move them around the house (which I do quite a bit), but always plugged them back when I am done. At night they generally all sleep plugged in.