They are designed to seal without tape or dope, and may actually leak if tape or dope is used.
You shouldn't use teflon tape on a flare fitting it will/may not seal correctly. The flare doesn't seal at the threads but at the flair and you don't want nothing to stop or get in the way of the connection such as a clump of tape.
-Good point, that's why I only use a little Teflon based pipe dope.Gas connections are far different than those of water lines.
Here also, should have mentioned that. The dope isn't to seal but to allow that little extra turn from the brass on brass that sometimes get sticky. My background,Started out as a refrigeration mechanic, then added HVAC and plumbing. Did some minor electrical work with a friend who was licensed. Had my own Mechanical Contracting business for more then a decade until Illness and Divorce did that in. There was nothing we didn't do, if we couldn't do it we knew who could, secret to our success, never say we can't but we always can.Then was a Director of Maintenance for a Group of 17 or so Nursing/Rehab homes.Then illness did me in again. But still putter around and try to keep up with latest technologies for the home.
Like you, I too putter, or more lately, mutter about the things I used to putter with. About 35 to 40 years ago, I did gas installation in residential, as well as propane tank install for homes and business.One thing I know and I know you know, but gets neglected in these hints and tips threads, is never mix metals, especially gas if avoidable, it creates electrolysis.Basically it is a battery and battery's slowly degrade over time by giving up electrons via ion degradation, in the same way lead decomposes in a battery if not recharged.One of the opposing metals will fatigue and fail leaving a serious gas leak hazard.One of the worst mixes is galvanized pipe to brass fittings.