My daughter and I need to get from Cochem to Strasbourg on June 19th. Do I need to pre-purchase tickets prior to leaving the States or can I just buy them that day at the station? Same question for Strasbourg to Bern three days later. I have always just bought same day tickets to be flexible in other places, but going country to country, I wasn't sure. I do have a Swiss Pass already. For Basel to Bern. Thx!
You don't have to prepurchase tickets, but if you do you can get Savings Fare ticket from Cochem to Strasbourg for as low as 39€/adult (vs 76€ full fare).
If you book Offenburg (across the river from Strasbourg) to Bern on the Bahn website, you can get prepurchased tickets for as low as 19€, vs 66€ full fare. Strasbourg to Offenburg is 8,50€ on a local train.
Actually, Strasbourg to Kehl (right over the river from Strasbourg) is 4,10€, and you can get prepurchased tickets from there to Bern for 19€.
As you can see, flexibility has a price.
Check train schedules to see if there's one departure you really prefer and want to commit to, or whether several options would fit your plan. How to Look Up Train Schedules Online gives you the DB train schedule link and tips for using it. As you said, your Swiss Pass kicks in at Basel, so you don't need to buy a ticket or reservation past that point.