When is pipe dope/tape needed?
The only place tape is to be used is on threads, to be exact, tapered threads, the kind that are on pipe nipples, or the male threaded ends of valves for example. These are the kind that get tighter as you twist the parts together. If you look at the threaded end you can see the pipe is smaller at the end than farther down the length. Pipe dope can be used on these threads too. Pipe dope, otherwise can be used on the various types of compression fittings, the type that use a ferrule. When using metal pipe the type of pipe dope is not that critical, but if using plastic, be careful, there is pipe dope NOT for plastic.

In a nut shell, tape is for tapered threads only, dope can be used too. and either must be used or a leak will occur. Dope is to be used only on mating surfaces, flat, smooth or rounded. It is mainly insurance in my opinion, and I use it too, but the mating surfaces should make a tight fit on their own. I have had a many a leak before using dope, now the leaks are non-existent. Then again, maybe I just got better at it finally....