Yes, it is interesting how regional it is! I lived in Tokyo and in the north, and sumimasen (or suimasen) was basically just used for sorry-thanks (along with all the other excuse-me type situations). Also doumo was the least formal way of saying thanks, not arigato, unless there was some other way of demonstrating appreciation in which case it might be skipped altogether. Some of the people I was with would say sugoi about every 10 seconds and if you did something deserving a doumo you might get a sugoi instead. I wouldn't say doumo in a shop unless I knew the people, and I wouldn't say arigato much with people I was close to. I certainly wouldn't use arigato much at dinner, let alone doumo; with friends it was close to saying "I'm grateful" in English which you only really say if it's at least somewhat important.