I didn't have any problems in Rio with street crime, but it is an acknowledged fact of life there. Just take your usual precautions and you should be fine. What I do, for example, is to take a couple credit cards, but once I get there, I split them up (hiding one in my coat, bag or hotel room). That way, if your wallet gets lifted, you're not strapped. And definitely, definitely, call your credit card companies beforehand. Not just to let them know that you are going and not deny charges there, but also to get the number to call if you do lose (or have stolen) your card...
I wouldn't stress out about the crime, there, though. Go, have a good time -- just use common sense.
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."