It's definitely not good for the racquet to sit in a hot car or whatever for extended periods of time. But with the modern frames (i.e. not wood) warping shouldn't be a huge issue. And yes, the strings are where you'll notice problems before the frame. If it sits in a car trunk for a whole summer, chances are the next time you bring it out, the strings will be pretty dead and need replaced.
Be aware, though, that when anything heats up, it will expand. And when it gets cold it will contract. That's just what things do. And the frequent expansion and contraction could make the frame feel less responsive quicker than if it sat inside all day. The worst thing for the frame would be to leave it in a trunk when the temperature has a large range from low to high. If the day starts at 40, gets up to 80 (much hotter in the trunk), and ends back at 40, that's some pretty good stress on the frame. Granted, temperatures don't generally vary that much during a day.