From what I've learned from locals, filling tax declaration in Germany is fully optional, until you have your company. Most people are doing this, because they get a tax return. But the only consequence of not doing this is not receiving tax return.
However, what about the situations where you are an expat (arriving within tax year) or an ex-expat (leaving within tax year) and in given year you have an income in 2 (or more) EU countries? Does filling a tax declaration remains still optional?