When you click on a file that is not the first file in an epub and choose "Mark ___ as cover page", a popup dialogue appears:
"___ is not the first text item. You should only mark the first text item as cover. Do you want to make it the first item?"
The options are "YES" and "NO".
I've seen this popup some few hundred times, and always clicked "YES". The addition of a "Don't show/ask this again" checkbox would be, like, THE GREATEST TECHNOLOGICAL ADVANCEMENT IN THE HISTORY OF EPUB EDITING. Or, you know, a most welcome addition to Calibre's time- and energy-saving capabilities.