I've traveled thousands upon thousands (!) of km, and cannot remember the last time my ticket was not checked on a long-distance service, like those that run between FRA and Mannheim. It's possible to travel on local or regional services and occasionally not have your ticket checked, but I certainly would not count upon it. (In fact, I recall stumbling onto a 5AM train toward Frankfurt from Mannheim on a Sunday many years ago, and even at that hour was awoken from my slumber by the conductor asking for my ticket.)