Agreed, I've only been on one ICE train where the ticket was not checked, and it was a short segment. The RE trains somewhat infrequently, and the U-bahn, S-bahn and strassenbahn almost never (but once in a while).

On the local trains expect a pair to enter the train dressed normally, and then when the train departs the station they will pull out their Ausweise and canvas the car. Woe be unto you if you are caught schwarzfahren; it will cost you 40 euro on the cheap trains. Given the relatively cheap day tickets that are good on the entire city network, it only makes sense to buy a ticket (and make sure that you validate it).