If you check you bags through to Rome, there is really no "formal" customs. They made us go through a short security/passport control line where the passports were stamped and we were off to the gate.
We did have to fill out the Customs form on the the airplane and that's what they took in the security line.
Make sure that you have plenty of time between flights in CDG. At least two hours.
If you have the choice, go through Barcelona.
Once you arrive in Rome, that is where you go through customs.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08