As for threads on shower head, shower arm, etc, you can use either. One other place to use it (either) is when you're putting together the parts for a sink drain (other than the trap and any slip fittings). If it's PVC, make sure the thread compound say it's ok with it.

The biggest mistakes by users of teflon tape is wrapping wrong, wrapping in the wrong direction and number of wraps.

For 3/4 copper female to PVC male: teflon or pipe compound or? If teflon, approx how many wraps?

When PTFE tape was first introduced into the plumbing industry all they made was the thin ½" wide "single density" tape such as we find on the small blue & white or red & white $.98 cent plastic spools in the hardware stores. Shortly thereafter they began making ¾" & 1" wide rolls, which were principally marketed through the Plumbing & HVAC supply houses. In the early 80’s they began making a "Double Density" PTFE tape which was twice as thick as the single density tape, however, here again it was primarily only marketed direct to the Plumbing & HVAC industry through the trade supply houses.

Shortly after the double density tape was introduced the National Gas Code was amended prohibiting the use of single density PTFE and requiring the use of pipe dope or double density PTFE tape on gas lines. Needless to say, even though the Double Density PTFE tape was approved for water service nonetheless it was soon referred to as "Gas Tape".

The ASTM (American Society of Testing & Materials) has now amended the standards for PTFE requiring 3 layers of "Triple Density" PTFE tape to seal threads on pipe from 1/2" to 2" in diameter.

In addition, they have now adopted a color-coding system to define the different types of PTFE tape.

White –--Single Density for use on pipe threads up to 3/8" diameter
Yellow---Double Density- Gas Pipe
Red---Triple Density for use on pipe threads from ½" through 2"
Green----Certified Oil Free- for use on Oxygen and Medical gas lines.
Copper---Contains copper granules- May be used as a mechanical thread lubricant but not a sealant.

I prefer to use pipe dope on all threads that are intended as a permanent joint or threads that will ultimately be in a concealed location and I use PTFE on joints which we can expect will be changed periodically such as shower arms, shower heads and angle stop valves that have a threaded input port.

Now in regards to the Copper x PVC connection

Quote; "Personally copper male to PVC female has always been the better transition."

There is only one minor problem with that idea. The plumbing Codes expressly prohibit any Female threaded PVC fittings.

"No female threaded PVC fittings"
Ref; IRC-2904.16.2 & UPC-606.2.2

I'm a gasfitter only, no plumbing. When I thread brass to brass I use dope only and prefer Rectorseal. When I thread black to black or black to brass I use about three wraps of single density tape and pipe dope. When I thread anything black for use in liquid piping I use dope, then tape, then dope. I personally have found these methods to be leak proof.

Thats true LP, but maybe they should prohibit these overseas black iron fittings too, some of them are almost egg shaped it seems.

I have used the hardening type of Rectorseal on 6" saturated live steam lines firing at 1000psi (boiling temp 546degF)

If its too messy your applying to much dope and if you not getting a good seal you simply are not tightening the joint enough. When an NPT joint is properly tightened you should not be able to see more than 1 turn of thread on the pipe.

If you see more than one turn of threads exposed at the joint, get a bigger pipe wrench and torque it down.

For the 3" pipe mentioned above I always resort to my trusty old "Rigid" 60 aluminum pipe wrench.