All depends on your fare and class of service. Full fare J and F are usually the same wherever/however you buy the ticket, and have no advance purchase requirements so you can hold or waitlist seats without buying a ticket and just pay for it at the airport when you check in (have done this many times on SQ). Obviously, web specials, discount economy fares, 2-for-1 J sales etc. have to be booked and paid for ahead of time.