Having lived in Caracas, Venezuela for 9 years, I can confidently say that Salto has a minimal problem with crime and danger. It is sometimes difficult to enjoy travel in some of the larger cities because of the real possibility of becoming the target of a thief, but in Salto, tourists can pretty much walk around, even late at night, with out much worry.
Obviously there will be a few places that would not be recommendable to visit, but the normal tourist will never even see those areas. As a matter of fact, the Salteños almost invariable comment on how calm things are here, and they are right. Tourism in Salto can be a welcome time of beautiful relaxation.
After 11 years of living in Venezuela (most of that time in Caracas), it is hard to find anything dangerous about Salto. As a matter of fact, one of the most frequent comments heard by the people here is that Salto is muy tranquilo (very calm), and you can't argue with that observation!! But, that is really one of the greatest aspects of Salto - you can enjoy a great vacation and not be overly-concerned for anything you might be carrying.
There might be some places where you would rather not go at night, but the normal tourist will never see those places. So enjoy a South American vacation without the headache of the constant impending danger found in some of the larger cities.