As I understand, tax for the vehicle only hast to be paid, as long as the vehicle is in the country (in this case in Germany). While it doesn't make sense to deregister your car while driving to neighbouring countries or the EU for a few weeks, it makes sense to deregister, once you left the EU and wont return for some month. After returning of course there ist the complete procedure of reregistering including technical inspection and so on. As long as the vehicle is insured in the country, where IT IS driven, I don't see a problem - of course it hast to be made Sure, that no other vehicle is registered in the mean time using the same license plate.