"Unfortunately, dishonest employees are everywhere, and I certainly would not consider myself a 'whistle-blower' in the workplace. If I found that a co-worker was stealing, or doing anything to harm the company, I would approach them directly to ask what was going on. There is a good chance of the situation being a simple misunderstanding. If it wasn't a misunderstanding and the co-worker was outright defiant with their damaging actions, then I would bring it, professionally, to the appropriate parties."
"If I were made aware of a co-worker being dishonest or stealing, I would report the situation right away. There is no sense in aggressively approaching someone when they are doing wrong in the workplace. It's better that HR or the appropriate party handle these situations."