Oh yeah, I have some European friends that are the same way when they come here, the know when I am in the car with them though its top up and air conditioning on

Of course you have that option and it might be way cheaper then if you reserved it upfront.

However, you also run the risk of not being able to try to upgrade or it costing much more then renting the car you want in the first place.

The moral of the story is to already have a reservation for a class of car you know you are fine with and if something else better works out great. If not, you still will enjoy your trip too!

Hope this helps.