I think leaving them out depends on the dog. Roxie is left out and never makes a mess. In fact, the only times she has gone potty on the floor since she got trained was the day I brought her home after being spayed. She walked in the house and pooped on the floor. I think it was her way of saying, "Take that!" Trinity, on the other hand, has to be in her crate when we are gone. She will get in the garbage and strew it all over the house, then hide when we come home because she knows she was naughty.