It's always been my understanding that cross tuning was accepted in early Cajun music even before accordions showed up, so I'd imagine pretty early on once the C accordion came into the picture.
This is very interesting. D would be the natural key to go with the fiddle. The forces that moved the standard to C are probably complex, but have to do with changes in music and also a need to lower the singing voice. Fiddle tuning followed along with it. The fact that much popular music from "english" traditions were in C and G certainly had an influence.
I do know that when playing by myself, I tend to prefer playing my D box, actually playing in the key of A most of the time.
Gurgley...that works for a description. The D box does seem cleaner etc. Of course, I like irish tunes which are most commonly in D or A anyway. At times, I will close the low reeds on my C box. In fact, I think the old players messed with the reed combinations more than we realize. Listen to the "Creole Belles" and you will hear several different accordion "sounds", from low and growling to clear and high. If you have a fairly wet tuned box, closing one of the middle reeds works well also.