I also never had problems with no-shows on the inbound flight and never was charged retrospectively.
Once, I wanted to change my booking of a VIE-FRA (return) flight that was booked in T-class. This would have meant to pay the fee for rebooking (75 EUR) plus paying the difference in price to the next higher booking class (~50 EUR), as there were not seats available in T-class anymore. Therefore, even the LH call-center suggested to me to book a completely new ticket (with the benefit of having an extra outbound segment to use another time). I asked whether I should cancel my original inbound segment. This appears not to be necessary.

P.s.: Thanks for the tip to claim taxes, fees, surcharges for unused segments.