We did 3 weeks for our honeymoon (9 nights WDW/UO, 4 nights New Orleans, 5 nights NYC and 3 nights Las Vegas) and took a fair amount of cash, all $50 and $100 notes.
I wouldn't be changing over too much in smaller notes because as soon as you break a few, you'll end up with a heap of these smaller notes, and its quite cumbersome.
In WDW, we used the magic bands almost everywhere so that came off the travel card, and any of the larger purchases went on that, otherwise, all cash.
My sis and her hubby also did 3 weeks in the US (10 nights NYC, 4 nights Las Vegas and 10 nights Hawaii, nothing Disney) and they took USD$10k cash, zero card action.
If I was doing WDW again, EVERYTHING will go on the magic band, it was so unbelievably convenient!! I want a Magic Band for my life... car key, house key, credit/debit card etc...