I have to book my ICN-NRT flight to catch my NRT-LAX connection on JAL. Assuming no IRROPS, is it fine to book any carrier that operates the ICN-NRT route? And further, do I need the NRT-LAX ticket printed at ICN to board my flight? Been a while since I've done this so appreciate any recent DP. Won't be checking bags, if that matters.

Last year I flew ICN-NRT on OZ and NRT-ORD on NH. The OZ desk at Incheon printed my onward ticket but I never figured out if that was really necessary.