If dropout spacers don't come with your travel case or you are just shipping a bike in a cardboard box, you can make excellent spacers from trashed hubs. Ask your LBS for wheels that have been damaged beyond saving or hubs with bad bearings. Most shops have a few of these lying around and will be happy to give them to you.
To reduce bulk and weight, I remove the axle, cones and locknuts from the hubs and throw away the hub shell. Reassemble the cones and locknuts on the axle with the proper spacing and hold them in the dropouts with your skewers.
You can use nutted hubs too but you need the proper size wrench(es) to put them in place and remove them so QR hubs work better.
BTW, don't rely on the plastic slip-in spacers used to ship new bikes. They don't fit tightly enough to stay in place for the type of shipping most of us do. I found this out the hard way.