There are cuevas in Bariloche, so I assume that if you go to a similar size city there will be cuevas in the more touristic areas. You will receive a lower rate than La Nacion publishes, if that is lower than what you accually get on the street in BsAs I don't know.
You will not be able to trade 20s, maybe not 50s, best is crisp 100s.
Naturally you gamble either way, if you trade in BsAs maybe the rate will be higher a month later, or lower. Nobody knows.
To keep in mind, if you are going to carry cash, dollars are smaller and easier to hide. But you will have to think ahead all the time so that you do have the correct currency. You cannot assume that you will be able to pay with dollars for accommodation, but maybe you can. at a lower rate.
What a nice trip you have planned! Have fun and take care of yourself and your money.