I've flown with bike to CDG several times. I much prefer to assemble my bike(s) in the terminal and
ride to Paris proper. Unless you travel light with a folding bike, I would advise against taking the RER. Reaching the station at CDG will be a challenge. Getting out once in Paris (Gare du Nord/Chatelet/etc.) a nightmare.
Paris is quite bike friendly. Plenty of paths. Plenty of commuters.