No. In fact, Swiss trains never "sell out". You might not find a seat, in which case you stand until a seat opens up. The website www.sbb.ch/en has icons on the schedule indicating expected loading for each train in the near future. If you are worried you won't find a seat, you can purchase a seat reservation for 5 CHF. So as far as ticket purchases go, buying far in advance gives no advantage over someone who buys a ticket while standing on the platform with their smartphone seconds before the train arrives, its all about the seat reservation.
If you want to get a discount by purchasing a "Supersaver" ticket, you can get a discount buying ahead of time. These are train specific, like airline tickets. You must ride the train shown on the ticket as it is not valid on any other train. Or you could buy a "Saver Day Pass". For a fixed price, you can ride unlimited trains on a given day. These go on sale 60 days before your intended date of travel and prices go up as your travel date approaches. Earlier you buy, the lower the price. They are nonrefundable.