Use TFE tape ONLY on NPT or BSPT threads in a steel to steel joint (or steel to iron.) Never use tape on any other type of thread and never, ever use it with aluminum, zinc or stainless steel. When in doubt don't use it, rely on a TFE filled pipe dope.
All other thread forms used on fittings have some kind of auxilliary seal like an o-ring or a doty seal which is all you need or want although a TFE-filled dope can give an extre measure of security and helps lubricate during assembly. The 2 tapered thread forms need something to fill up the spaces to prevent leaks, therefore TFE dope and tape. The problem with tape on Aluminum stainless and zinc is that it can actually promote galling and make disassembly/reassembly impossible, not to mention ruining the parts, TFE-filled dope causes no such problems.
One more thing. Never use tape anywhere where small pieces of the stuff getting carried downstream can cause a big mess by clogging vital ports, etc. Basically, use tape when you re-plumb the crapper and not when you fix the car.
