I've previously flown QR in Economy and I am due to fly CX for the first time in Premium Economy. Both airlines allowed me to choose a seat at the time of booking without charge.

But back to the OP's query, you may get away with it. Certain routes such as DXB are always full from my experience. I'm due to go next week and all window and most aisle seats were already snapped up a while ago! I tend to keep an eye on the seat selector when opting to pay for seats in order to get an idea on how busy it is, though some seats may open up at t-24.