The FCOM4 states that on visual approaches the landing runway should be inserted on the active flight plan, so what to do?
Where does it say that you should? All I can find is: "When flying visual, the pilot
may select an appropriate STAR and RWY in use on the MCDU"
FCOM 4.05.70 p29
No reference for sidestep.
For a normal visual approach it makes sense that you select the landing RWY but don't make it too hard on yourself in case you have to do a go around without the correct routing in the box after a sidestep.
You are ofcourse free to have whatever you want to have in the FMGS.