Emphatically, no. You will likely get assigned a seat during check-in (though you should still skip it if the airline tries to tack on a fee) or, at the very least, at the gate. Again: You don't have to select a seat in order to get a seat. You just might get stuck with a suboptimal one if you choose to skip it.

Yes, most airlines allow you to choose or change your seat during the online check-in process, which typically opens 24 hours before your flight's departure. For many "Main Cabin" or standard economy fares, this is often the first opportunity to select a seat for free if you did not pay for advance selection. However, the availability is limited to whatever seats are remaining; "Preferred" seats with more legroom or those in the front of the cabin may still require an additional fee. If you are traveling on a Basic Economy ticket, the airline may have already assigned you a seat automatically, and changing it might not be permitted or could incur a high cost. For international flights, seat selection during online check-in is a great way to ensure you are seated with your travel companions. If the seat map appears full, you can often check again at the airport kiosk or with a gate agent, as airlines sometimes "block" certain rows until the final hours before boarding.