The rule still exists but with the additional requirement to buy an "initial" ticket from a vending machine if there´s no ticket counter availlable or if it´s closed at that time. If there´s no vending machine (can´t imagine any railway stop in Germany without one) then you´re supposed to report to the train guard immediately after boarding the train.
Otherwise one is subject to pay the "service charge".

It is advisable to have Euro in cash to pay for the ticket if using a ticket vending machine as those located at small stations/stops mostly DO NOT ACCEPT ANY CREDIT CARDS (they usually accept only EC debitcards which foreigners barely hold).
It´s wise not to rely on CC acceptance at vending machines in Germany at all - regardless where you are - CC´s are not commonly used for that purpose in Germany)

Nevertheless you will not have any problems at Saarbrücken Hbf to get your ticket since Saarbrücken Hbf is sort of "major" station...
If you need to buy your ticket at the vending machine there´s a list of "surrounding" places on it. And Landstuhl is most certainly listed there anyway because it´s within a radius of ca. 50Km´s from Saarbrücken. Just enter the corresponding numbercode shown on the front side (4-digits i think) and choose adult or child fare and pay... That´s it !