It has been my impression, having spent a significant amount of time in Germany, that German ticket checkers aren't primarily trying to detect people who have made a mistake, but those who intentionally ride without paying.
This information may be a bit dated, but I visited Berlin circa 2008 and my friend (American expat) who had been living there for 10+ years insisted that we not pay at the subway, because no one checked. Following his guidance, I never paid for the subway, and was never fined. This was just within the city of Berlin, though.