AS BHunter mentioned, a controlled temperature is always better for storing bows. Heat and cold will expand and contract materials--a hot vehicleis certainly not good for storing a bow--bows have been know to 'explode'. As for the cold, well the strings turn a bitand of course mother nature temperaturesdoes change the way materials react.
Keeping it in a vehicle, one will have the chance of the bow getting ripped off also--a little more effort will sometime go a long way. Find the extra time and store your bow in someplace where it's safe--it's better to be safe than sorry.