For best performance, create the CEGUI elements only when they are needed, and destroy them when they are no longer being used. Keeping unused elements in memory can lead to performance issues, especially if the player is likely to use them a few times only.
However, if you have a lot of CEGUI elements in your window that the player would frequently use. In such cases, it's more efficient to hide/unhide the elements instead of creating and destroying them each time.