Someone feel free to correct me if i'm wrong, but...
I think when the icon for charging switches from lightning bolt to the plug it is letting you know that the iPad is in a trickle charge mode, which means that the iPad uses the power coming in to keep it just at 100 percent without fully charging and harming the battery.
As far as giving your battery a workout, I agree. You should always strive to keep your iPad battery running until you get below about 50% remaining, then do a full charge overnight. While charging it after everytime you use it (ie. getting it down to 60/70/80/90%) shouldn't hurt your battery, it is best to flex its use and keep it going longer than that.