Question on tipping

We didn't tip ours and he didn't act like he expected one.

No, no tipping.

I was in Greece last year, and it was so stressful figuring out whether to tip drivers lol

In general, you don't need to tip a person who's delivering a service according to an agreed rate.
If the person has been very helpful or went beyond the service (e.g., carrying bags to your accommodation door, showing sights instead of just driving...), then yes, you can tip, but only if you want to, and not excessively. No more than maybe 5% of the initial cost, and in any case you need cash to do so.
In your case, for a drive from CDG to Chartres that probably costs 200+ euros, if the driver's nice and if I have a 5€ (or 10€ if they've been unusually helpful) note in my wallet, then I could tip that. But if I have nothing, tant pis and I won't feel bad.

If you take taxis later on, it is customary to round up to the nearest € when paying cash, but mostly as a practicality regarding change. Card payment won't let you do so.

I booked a car service to take me from CDG to the Montparnasse train station and was thinking of tipping him or her five or so Euros, depending on the experience. It's not required or expected, but is appreciated.

Many thanks to all of you. I still remember the tour guide on my first Rick Steves tour telling us that we were
not in Italy to change the culture as far as tipping was concerned.

This is quite an expense but after travelling overnight from Boston as a solo traveler decided this was my comfort
level. So looking forward to this amazing itinerary.

Quite the expense, but really worth it! Hauling yourself from CDG, to Montparnasse station, then to Chartres is a bit of a schlep.
By the way, perhaps other travellers on the same tour could be interested in sharing your ride? I do not know anything about RS tours, but maybe there's a way to reach out to other participants?

We do get a list of tour members but only by first name. I will check with RS office to determine if there is
someway to communicate with the other participant from Boston. I am arriving 2 days early as I have been
looking forward to the Chartres en lumieres for quite a while. Of course excited about the Cathedral and the
town itself.
Thank you balso.

Even Rick says sometimes it just makes more sense time and comfort wise to take the taxi, in spite of his usual recommendations. It's your vacation, spend your money as you see fit. Some spend it on food, others clothes, or museums, or to make travel safer/easier/more comfortable.

I still remember the tour guide on my first Rick Steves tour telling
us that we were not in Italy to change the culture as far as tipping
was concerned.

Our guide for the Loire tour told us pretty much the same thing, she said everyone's happy to take your money but it's not necessary.