Should I prebook a train ticket when travelling within Switzerland?
Yes, if you have a definite plan for a certain date and time. There are super saver tickets available for up to 50% discount but you have to buy those tickets in advance, preferably at least a week earlier. I have personally traveled to Switzerland twice and found this a lot cheaper than regular prices and Swiss pass. The only concern is super saver tickets are non refundable and if you miss that train, it would cost you more to get another Swiss rail ticket.
Source: Swiss Super Saver Ticket
And the price for your desired journey is around 36 CHF as a super saver ticket, but you have to buy them at least a week earlier. You may be able get a super saver ticket on that particular day, which depends totally upon availability.
Note: In the mobile app (which is otherwise quite decent) not all available super saver tickets are displayed (only the ones for the quickest routes). So on the online webpage you might see even cheaper options which aren't shown in the mobile app. Relevant news article (german): http://www.srf.ch/sendungen/kassensturz-espresso/sbb-sparbillette-im-internet-spart-man-mehr-als-mit-der-app
Geneva to Interlaken: As an example for next week, on the 12.01.2017 there's a ticket available for 36 CHF from Geneva to Interlaken.