The tapered pipe fittings we use here have what is called an interference fit. The threads will leak once you have a good mechanical connection - you need something to fill in between the threads so it can't leak around them. Pipe dope flows, and if you apply it properly should work fine. Teflon tape makes it easier to thread things together to make the mechanical connection, but if not applied properly (not enough wraps, or not the proper direction), it won't fill in enough to block the flow past the threads. So either one by itself should work. Some people use both...one helps back up the other.
The quality of threads on the fitting can affect how easily it can be sealed...many of today's fittings have really lousy threads - torn up, made by a dull cutter, and may not be to spec, so sealing is harder.