Ok, thanks for the lesson and all the opinions. I knew about tapered and non-tapered screw fittings but did not relate the two types to the use/non-use of sealants. I've operated under the principle of if the matings are metal-to-metal, use a sealant. If plastic-to-plastic or plastic-to-metal a sealant is redundant because the plastic is compliant enough to "fill-in" any irregularities in the machined surfaces. After reading Jim's explanation, I've gone back and re-read the instructions of my last plumbing experience: installing a kitchen faucet and there is NO mention of having to use a sealant for the supply fitting to faucet fitting connection!! However, HJ says he ALWAYS uses sealant at a flare connection so I plan to go with my initial inclination and use a sealant carefully and sparingly at the flare end. For other future water plumbing projects however, I will experiment and skip the teflon tape and rely on the flare ends, compression ring, rubber gasket, etc. to give me a leak-free connection. For gas connections I will use a sealant for the extra insurance it provides. Thanks again for all the inputs!
Alright, I may or may not be an expert. I have worked with many greats, many good enough, and a few of what I call the greatest of their craft through my life. There are many types of goop, goops for all needs, each having a use, in a time and place. You must know your goops and where to use them. Teflon tape is used like a goop and the most incorrectly used item on threads that I have ever ran across in my life. Loctite is a wonderful goop that I may have only used less than one hands worth of counts or like twice, as it shouldn't be needed in almost nearly all settings (the sever loctite not the loctite brand). Loctite brand, just to say, does also make many wonderful goop. There are certain goops only available in certain brands from certain suppliers, that one day you may take a personal liking to one goop over another from suppliers and wholesalers, Dap is an interesting company few use regularly.
That all said. Plumbing is where many of the goop are most incorrectly used, and I am still learning goops. I know how and when to use, and most importantly, when not to use teflon tape. That is the key to get down. Yet, there is the temping, "See if Teflon Tape will stop the leak," error of so many. Teflon you seem write on, as metal to metal threads, most commonly, unless the fitting is compression type variation. A compression type variation could be flange ring connection of many variations, the rubber gasket internal flange, you will find on the faucet connection lines, to sinks, toilets, and such. Your garden hose doesn't probably have a compression class of connection, as most don't, yet use a flat washer that gets compressed to seal, yet allows for easy disconnection and re-connections. Compression fittings are often rarely changed or disconnected, yet are used in places of disconnect and re-connections without the Teflon or thread treatment needed if done right, nor recommended. Plastic plumbing line should never really need Teflon Tape, yet rarely, it might help, if used on the right connection type of plastic following the rules of Teflon Tape, until the part can be replaced, due to Teflon never really seeming to provide a good fix to plastic thread items, like metal to metal. Teflon is used where the threads are part of the seal against leak. Again, Teflon is used where the threads are part of the seal against leak. Compression fittings are designed where the threads provide torque to compress the bevel, farell, or ring or such that holds from leak. No Sealing agent or compound, or Teflon Tape is needed in this case, or could mess up the torque.
The most common plastic connection that is sealed with Teflon improper is drain systems under sinks of plastic parts, yet on one rare type of system, there is one location that holds a lower pressure (all drains are low pressure, yet a back pressure location) point with the threads. Teflon, may or may not be allowed in this location, yet shouldn't be needed still.
Gas line goop, is essentially the same, where the thread holds the leak, goop away to manufactures recommendations, where the fitting is compression, the manufactures of compression fittings have designed their fitting for no goop. This sometimes means making some compression fitting connections two or three times to seal without leaks, using a few extra parts at times.
I don't know if this helped, was read, or is just one big wasted ramble.