The Precautionary Principle is the ethical theory that if the consequences of an action, especially concerning the use of technology, are unknown but are judged by some scientists to have a high risk of being negative from an ethical point of view, then it is better not to carry out the action rather than risk the uncertain, but possibly very negative, consequences. [...] The Precautionary Principle is often applied to biological fields because changes cannot be easily contained; they affect everyone. The principle has less relevance to contained fields such as aeronautics, where the few people (e.g. test pilots) undergoing risk have given informed consent. (from wikipedia)

Winners of elections are allowed to take decisions which grately affect everyone, thus I think to ensure that elected politicians really represent the will of the electors' majority.

The Precautionary Principle applied to elections means that even the smallest doubt about e-vote being really less risky than paper voting it's a good reason to use the traditional paper ballots! By the way, what's wrong with ballot paper?

The European Union applies the Principle in the following fields: Consumer protection, Environment, Food safety, Genetically modified organisms, Public health.
Thus EU should apply the Precautionary Principle to elections because

• when we vote we are "consumers" of democracy
• winners of elections are allowed to take decisions that greatly affect both the natural environment and the social environment (which for humans is often more important than the natural one!)