We've all heard of "new" tires, but it's not how much tread is on them or how much they are not weather cracked, it's the production date that's on the inside sidewall serial number stamping (or used to be). Even if a tire is lightly worn, but many years old, when the rubber ceases to look like what it does on your everyday vehicle and about 30,000 miles on those tires, new tires probably need to be a consideration, just like making sure the spare tire has 30psi in it. When the look of the rubber gets dull and its obvious the tires were installed several years ago, the rubber in the tread does not look "newer" with use, and there's a vibration that you can't "drive thru" like a balance issue, put new tires on the "get list" and be done with it.
When I bought my '67 Chrysler, it had some Montgomery Ward bias ply tires on it (this was in 1981). They looked decent and they drove ok. I was on the way home one night and had to swerve and hit the brake to avoid something in the road. A few miles later, I noticed the hood leaning to one side--and it was getting worse. I finally stopped near a repair shop that someone was still working at. I pulled onto the shoulder and turned on the flashers and then looked at the tire. I walked to the shop and got them to put the pristine spare tire on the ground for me. Then I gently drove home and ordered up some fabric belted radials of the correct dimensions for the car later that week. A total of 13 miles on that original Goodyear PowerCushion tire and the casting flash is still on it.
So, even if it's not a flatspot or out of round situation, it can be just a sudden maneuver that causes the loss of air. In the mix of figuring tune up and other maintenance items as you go through the car you just bought, we all probably need to figure a few hundred extra into the mix for tires too.