The rules that I learned want me to file a flight plan whenever I am intending to cross a FIR boundary or when I plan to enter controlled airspace. Flying a (European style) ultralight I am not allowed to enter controlled airspace anyway, and fly VFR only.
There are a few exceptions: no flight plan is required for crossing between Germany and several of its neighbour countries, including mine. If I do, it is usually for flying into France - which is perhaps 5% of my flying.