It also depends on the location and the definition of "panel". The requirements for listing by an NRTL (Nationally Recognized Testing Lab, of which UL is one of many) vary from state to state and even by large cities. The National Electric Code does not specifically require NRTL listing, but implies it in many places.
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden