SimonG reply about someone having leaks because of no paste reminded me of a neighbour who used a guy who wasn't really a plumber to do a bathroom plus all the plumbing/heating in a new extension.
Neighbour said the guy asked him to get an engineer in, as he couldn't seal the valve tails on a rather odd new rad in bathroom. Went in to sort it & noticed the rad bosses were a bit oval, to be fair, but when we said it was better with paste as well as ptfe, the bloke (who couldn't sort it) had a cheek to say it didn't need it! I redone the ptfe & pasted the tails. No leaks & it is still sound at least 20 years later.