PTFE tape is used on tapered pipe threads. If the threads are parallel the sealing is by some mechanism other than the threads. For BSPP (British Standard Pipe Parallel) threads tightening the nut on the threads presses two pieces of metal against each other (i.e. the rounded end of the pipe and its mate on the tap) and that metal-to-metal contact is what seals the joint. Teflon tape on the threads won't do any harm, but it won't do any good, either. If there is a problem with the metal-to-metal seal the joint will leak whether or not tape is on the threads.