As noted above, monarchs fall into the second category. Over the years I have netted 'monarchs' flying among vegetation that turned out upon closer examination to be viceroys, queens, and on one occasion, a large gulf fritillary. The need for practice in species identification brings up another topic. I've even received photographs of a tiger swallowtail from someone who believed that they had photographed a monarch butterfly. In my opinion, voucher specimens are the better of the two options.
Photographs can ensure that an identification was correct but present practical problems.
Monarch butterflies are abundant, large, showy, tend to roost in clusters, and while nectaring, almost seem to pose for the camera. Most other migratory butterfly species are less cooporative. They are usually less abundant than monarchs, less brightly colored, smaller, solitary, and camera-shy, tending to fly off as soon as you approach. Finally, because the top and bottom surfaces of the wings can be equally important for identification, at least two high quality photographs are required for many species. On the other hand, preserved voucher specimens can be examined at leisure and may even serves as resource material for future DNA analysis.