Are RER tickets between CDG (airport) and Paris good in both directions?
 

Specifically, my family is going on a trip to Paris, and I've worked out we will be making exactly 10 one-way trips between central Paris (zone 1) and CDG Airport -- specifically, 6 CDG>Paris and 4 Paris>CDG (the logistics are a bit complicated but this is what it works out to.)

I'd therefore like to buy the book of 10 tickets at €80 when I arrive at CDG, therefore saving €20 and time, but before doing so I just wanted to make sure that this wouldn't pose an issue.

Also, IIRC RER tickets to central Paris just say "Paris," so if we got on/off at Luxembourg or Gare de Lyon (both RER stations in Zone 1), that wouldn't make a difference, right?

Correct
 

Yes they are usable for either direction: http://www.ratp.fr/fr/ratp/v_154903/...e-destination/ says "Il est utilisable dans un sens ou dans l'autre, par exemple : Paris - Antony ou Antony - Paris."

Translated it says "The ticket is usable in one direction or the other, for example: Paris - Antony or Antony - Paris"

From the same page: "Les billets à destination ou au départ de Paris permettent d'utiliser la correspondance métro et RER dans Paris."

Translated: "The tickets with a destination or departure of Paris allow you to use a correspondance of the metro and RER inside Paris"

You can get out at any station in Paris and use either the RER or Metro to complete you journey. You can also start your journey on the Metro or any Paris RER station to get to the airport.

RER tickets are station specific and if you purchase 10, CDG to/from Paris tickets at once, you will receive a discount. 10 tickets will cost 80€, one ticket is 10€.

Something to consider - Uber is 45-50€ from CDG, and may be cheaper or comparable, depending on the number of people in your party.

Cabs are fixed price 50€ from CDG to the right bank amd 55€ to the left bank, no extra charge for bags.

Thanks all for your help!

I didn't know that there was a fixed price for cabs to/from the airport. Is a gratuity expected? (I have no problem paying one if it's expected, but want to factor in all possible costs to make a balanced time and money comparison on train vs. cab/Uber.)

There is no need to tip taxi drivers. The fixed rate from CDG to the Righ Bank, 50€, and to the Left Bank, 55€, includes up to 4 passengers. There is an additional 4€ charge each for 5th, 6th, or 7th passenger.

Baggage fees, as well as approach fees, have been eliminated.