I lived in Texas for a while, and discs could get warped in the car there, but it was a rare occasion, and really only happened if they were left in the sun with any kind of weight or pressure on them.
My solution was to keep a few emergency discs in my car for impromptu rounds -- I'd always have a Teebird/Firebird/Roc/Wizard (or something similar) in a durable (think heat resistant) type of plastic somewhere out of the sun (under the seat, in the trunk, etc...).
As long as you have your goto molds in the car, you can play a pretty good round at almost any course. It won't be 100% the same as if you have your whole bag, but it will be close enough for spur of the moment casual play, and if something does go wrong and your discs get tacoed or warped somehow, you're not out many discs and not out any of your goto discs.
Notice the geography of the people posting responses -- for folks who in cooler areas, they probably never have had any problems. For folks who are down South, the heat can get lethal. Heat coming in through the windows is absorbed by the interior, and the glass acts as an insulator. The temperature in your car get up to 200 degrees, depending on the temperature outside, the kind of vehicle you have, and how long it has been in the sun.
At 200 degrees, discs start warping all over the place. If you want to see this for yourself, place a disc in a pot of boiling water and leave it there for a few minutes -- that's what the temperature can be in your car.
So make sure that discs are out of the sun and that they are in some sort of stable arrangement -- carefully stacked in a box, face up on the floor, or some other arrangement that will help prevent gravity from warping the discs.
