Absolutely!  Heat is the #1 killer of ammo.  It causes degredation in both the powder and the priming compound.  And being in a car is one of the worst environments.  This is not to say that ammo is going to go bad after a week or two, but over time, it will lose velocity (powder) and ignition reliability (primer).

I recommend that folks who carry guns in their car replace their ammo at least twice a year: once at the beginning of the summer season, and once at the end of summer.  Obviously, if you live in very high heat conditions, you may want to add a third replacement in the middle of the summer.

-Troy