The page http://www.clubi.ie/plumbing-heating/gripes.htm states:-

"PTFE joint tape in the thread of compression fittings is a common
sign of uninformed DIY."

Where exactly is one supposed to put PTFE tape if not on the thread of
compression fittings?

Well, certainly not on the thread of compression fittings. A compression
joint works with a seal between the fitting and olive and between the olive
and the pipe. These are metal to metal joints. Some people wrap the olive in
PTFE, but this is unnecessary and could even cause leakages in the future if
the PTFE degrades and there is no actual metal to metal seal.

The thread on a compression joint is not part of the water seal and should
not have PTFE applied. The sign of a professionally done compression joint
is the use of no PTFE at all.

PTFE is used on BSP fittings usually with a jointing compound. Examples are
radiator tails and outside taps, where the thread IS part of the water seal.

Christian.