I just can't leave a form un-trimmed. I even trim the bottoms of my coasters. I have fun playing with depth and concentric rings, especially around my logo stamp. It's a lot to put on the underside of a piece but I just don't feel I've finished it if there's not something to see when you turn it over.
Sometimes I like to play with sharp undercuts and glaze drips too. It's safer than drips on a vertical wall.