Do you mean booking the tickets or reserving the seats while you already have the tickets? In Switzerland, train tickets are not coming with a seat reservation, they must be done separately and are for most trains optional. Swiss train tickets can be purchased 6 months in advance.
You can only make seat reservations for IC, ICE, EC, TGV, and RJX trains. So for your travel to Lucerne, you cannot reserve the seats on IR trains. And, I can tell you from my work experience as a train conductor, a seat reservation on normal IC or IR trains is not needed, there are always enough seats available; you simply get on the train and take a free seat.