I occasionally travel this route.
Munich-Ulm: The trains usually start in Munich, so you will find plenty of seats.
Stuttgart-Munich: Especially around weekends, you might want to reserve a seat. Trains usually have traveled through half of Germany, so there will be many passengers on board. Unfortunately, this also means that there is a (very) high probability of trains being late, so you might end up using another train, without your reserved seat, and lots of other delayed passengers. Check the DB app early and look for alternatives.
In general, I look at the train schedule to see where the train starts – just click on the train number in the connection details.
If you use DB’s online search (reiseauskunft.bahn.de), you will see an estimate right underneath the travel times.
Orthogonal to your question title, but given that you debate getting a reservation, I want to add the following:
I found those estimates on the website to be not very reliable and prefer the following rule of thumb: In the middle of the week during the day, seat reservations are often not necessary. On a friday afternoon and on weekends, you really should get a reservation. The above screenshot shows a trip on a friday afternoon, taken on monday morning. The same train on monday afternoon (on the day the picture was taken) shows "medium occupancy expected".
Especially with short trips (and I do consider the trips you are planning short) and if you’re travelling alone, you often get lucky finding a seat without a reservation with the following hints:
Credit:stackoverflow.com‘
4 Mar, 2024
5 Mar, 2024
4 Mar, 2024