Bahn Rail Pass and Reservations?

In Germany, you don't have to have a seat to ride a train (ex ICE Sprinters). They let people on until they can't close the doors (I've never seen that happen). Reservations are only available for express trains (ICE/IC/EC), not for regional trains. In 13 years, I have been on only one reservable train (out of 12+) that was SRO, and that was on a major holiday. Although all of the seats were taken, less than half were reserved. If I had bought a seat reservation when I bought my ticket, about a half hour earlier, I would have had a seat. Seat reservations are only 4€/seat, so if you worry about it not getting on a specific train, it's not that expensive to have a reservation. You can get reservations until almost train time at a ticket counter or automat, or you can get them online in advance at the German Rail (Bahn) website.

One caveat up front- I don't know whether or not you need to buy reservations for ICEs with a railpass. That being said, on this class of train, I always purchase a seat reservation. The reason is not so I can guarantee myself a seat. As Lee noted, there's almost always a seat available somewhere. The reason is because I don't like fumbling around, luggage in tow, looking for said empty seat. With a reservation, I know exactly which carriage to enter, and I can procede directly to my empty seat with little to no hassle.

Good point, Tom. On my last trip to Berlin, the reservation displays were not working. Thus, people without reservations couldn't know where the reserved seats were. Some were becoming quite annoyed by being bumped by passenger with reservations (printed or on phones) multiple times.

My DB rail pass was for first class. I never paid out for reservation and always found a seat except travelling from Dresden to Frankfurt (Main) at 7 am. I had to move 3 times because where I was sitting, someone had the seat reserved. Therefore, if you travel during commuting time (7 to 9 a. or 4 to 6 pm) between major cities I recommend reservations.