I use my cards all the time (as do Brazilians) and never really worry about it. Obviously you never let them walk away with the card. Any legit place will bring the reader to you and do it right there.
Personally, I'm more worried about the Cielo branded machines trying to do dynamic currency conversion and charging a 5.9% Gringo Tax.
The machines say something like: Press 1 for BRL or 2 For USD/Eur/Home currency(or maybe hte other way around). If the person operating the machine just presses the enter key, it will automatically select the home currency.