In Germany all trains are without reservation. You don't need one. You just get on whichever train you want and travel. Some trains to Austria and Italy /Eurocity/ also do not require a reservation. But if you are traveling by night train, then you must have a reservation.
Reservation is optional, if the train is full you will have to stand or if it is too full you have to leave the train for security reasons. Check bahn.com to know how full the train will be.
You CAN make a RES (2nd cl is 4,50, incl 1 connection) ONLY for IC/ICE trains (doing long-distance)-on shorter sectors there will also be additional all-stops regional trains that cannot even be RES. Can easily do online via bahn.com and even also for some trains at the DB-machines in the statioon (though you need to know the tricks for that).
I hardly ever do it, and travel quite a lot in and via DE. ONly on very busy days/times. But it all depends on your own mood and how anxious you are about the sheer chance it will not go as planned-then I can reassure you it will not, as DB-ICE are always late or cancelled anyway.
DB only runs IN DE-as soon as its train runs over border by magic it becomes a train of OeBB or SBB-even without some painters coming along. Totally incomprehensible for USAers, but thats how it runs here.
You CAN make a RES (2nd cl is 4,50, incl 1 connection)
@mcadv
Including as many connections as there are in your journey, as you have been told numerous times.
