On currency exchange -- I changed money in Mandalay and Bagan, and both of these were relatively flexible on the quality of the bills (I had all pretty clean bills). When I changed money at a bank at the Zegyo Market in Yangon, they were very critical of the bills, and rejected a couple of notes that I had that I'm certain would have been accepted at places in Mandalay and Nyaung U.

One other thing I noticed was that the places in Mandalay and Nyaung U offered better rates for <$100 bills than in Yangon as well.

I never tried the unofficial money changes in Zegyo Market, but they were quoting 900 Kyat/dollar. At one point I saw one with an extremely thick wad of $100 USD bills -- I'd estimate somewhere around $40,000 USD that he was just carrying around!