Best place to pick up and return car on trip from Paris to Loire Valley

If you don't need the car for the first nightif you can get to your room from the train station in Orleans or wherever you will be without a car, and if there is enough for you to do that evening on foot, then maybe you can save a days rental by taking the train and renting a car the next day. It's not a terribly compelling reason, but maybe it works for your situation. I cannot think of a single rationale for not driving straight to the airport, but maybe I lack imagination.

You'll have a larger choice of cars and 24-hour service if you stick to your plan. On the other hand, getting to and from the Loire is easier by train.
I wouldn't chance driving up to Paris the same day for a 1:30 flight. You have to be at the airport at 10:30. I'd turn the car in the night before and stay either in Paris or at the airport.

Rather than picking up the car at Montparnasse, why not take the TGV from Montparnasse to St. Pierre des Corps (suburban Tours) in the heart of the Loire and pick up the car there? It will be easier and probably faster, and you can easily get to any destination in the Loire from SPDC. There is, if memory serves, direct service from St Pierre dc to CDG Airport as well....

What's never made sense to me is paying for two modes of transportation on the same day, plus the time it takes to change between the two modes. You're looking at about fifty bucks for gas and road tolls each way. Paris to Orleans is about an hour and a half. Blois to Roissy is about two and a half. You need to get there three hours before the flight; it'll take you a half hour to turn in the car and get your junk close to the counter. Add another half hour for a traffic/lost cushion. You'll need to fire that sucker up by seven.

I think the train outbound from Paris makes perfect sense. Then drive back to CDG for dropoff. That way you don't have to drive in downtown Paris and don't have to complicate your departure as Ed describes. We did this last summer - took the TGV to St. Pierre des Corps (Tours), wandered around Tours on foot, had a fabulous meal, then picked up the car. We tootled around the Loire Valley for two days, then drove back to the outskirts of Paris. Very low stress and convenient.

Personally, I would do as Nelly suggests - and I would try to get to the airport at least 4-5 hours before the flight (cushion time to account for unexpected road delays or turnarounds).

I agree with PP who suggested training to a city in the Loire and renting a car there and then returning to CDG. A few tips to smooth your journey! * Train station car pick ups usually have fewer car options, ie, automatics and maybe GPS. After you reserve, the car company may send you an email saying the type of car you want is not available. I don't know if this is true in Tours. Make your reservation as soon as you can. * If you are used to picking up your car at a major airport, you will be surprised at the limited hours of opening for car pick up and off at train stations. Maybe not open after 6 PM, half day on Sat and closed on Sunday. So please work your itinerary around the pick up hours.
* If you are going to be renting a car elsewhere, you might want to consider bringing your own portable GPS with you. Need to buy the Europe add on. Trying to figure out a new GPS is , well, "trying". You will love Paris and the Loire. Perfect "town and country" option. Bobbie

Thanks to everyone for their great input! After mulling over the options we agreed with Ed about not dealing with 2 modes of transportation in one day. Plan is to pick up the car at Gare Montparnasse and drop at CDG. We will not be in a hurry en route to Loirre so armed with GPS and map (and a very good map reader) we will just take our time. Heading back on a Sunday we hope will be in out favour with roads not being so busy. Google maps shows it is 2.5 hours drive from Chateau to CDG so the plan is to hit the road about 6 am which will hopefully allow for anything (!) that might arise. Thanks again everyone!