Sometimes yes (without prior approval). In practice this depends on the size of contribution, purpose, and relation to your current employer's industry. I've found there are two types of people: those who do and ask later or claim ignorance, and those that ask up front.

I'd guess if you are employed somewhere with these draconian rules concerning unrelated side projects (i.e. pre-approval) then you might not be the type of person they want. I know that feels a bit like employee/victim-blaming, but it also shows how important that employee values his personal software freedoms.