Be very careful about holding any meat under 140 for more than 6-8 hours. 135 is in the danger zone for foodborne illness. Adding citric acid can sometimes help to eliminate or reduce some bacteria like e-coli and salmonella but it's no guarantee.
True, ive never had issue, but good food prep practices always play a bigger roll in my book. Besides that fact, if your steak has a ecoli or salmonella even before you cook it and your not cooking well done on higher heat, you already run that risk, no more risky than any other medium rare meat, touching raw meat packages at the grocery store, holding hands with a child that doesn't quite no the concept of washing hands, eating sushi... etc etc. But I'm a guy on the internet, so anybody willing to sous vide at medium rare temps, do so at your own risk.
