Dave, I always thought of scallops as the original custom paint job....and an extension of the way artists of the '40s & '50s used "speed lines" to suggest fast motion.
I do feel, that they might not have been the original style, but the long life, and undying popularity of flames has earned them the title of "the classic custom paint job" of this hobby.
After 35 years, flames have always been about 50% of my work. Today, even though I am focusing on illustration, and unique themes, rarely are less than half of my requests for flames. Right now I have 4 small jobs in process. One is muraled...one is egyptian art....and (you guessed it) Two are flames. After I get those done, I have another flamed '32 scheduled.
If I had to do all of the flame jobs just alike, I'd look for a new job, and do just the interesting stuff. :-(