Parents are constantly reminding their kids to wash their hands, especially with the flu and virus season coming. But how do you strike a balance between teaching good habits and becoming obsessive?

Hands quite possibly make more contact with the world than any other part of your body. Such contact often includes places where germs flourish like door knobs, handles, pens and, yes... keyboards.
So what makes something a "good habit" versus an "obsession?" Good habits are those that enhance someone and lead to useful outcomes. So, for example, practicing an instrument routinely can lead to mastery; and washing hands on specific occasions, such as after school, or before bedtime, can help maintain health by reducing the incidence of viruses.
An "obsession" is a belief - a modal operator of necessity - spawned from intense anxiety. That is, something "must" occur in order for something else to occur (often, the avoidance of imagined severe, unpleasant consequences). For example, "I must wash my hands many times per day, for at least five minutes per occasion, in order to be safe from germs." A good habit is forged within a framework of what is possible and relates to the development of something useful; as opposed to an obsession, organized as a"limitation" preventing the occurrence of something that is desired.  Instilling behaviors that serve a purpose, calmly, emphasizing the outcome that is served, rather than putting your attention on what could go wrong by failing to do so, I believe, is the balance point you are after.