There should be a copper (or 'Dowty' type) washer between tap and tank as they rely on a face seal rather than the thread (as Ian mentioned, they're parallel threads and not tapered). This would also apply to any adapters that you use.
The Fuel lines at the tap and at the carb banjo have a conical seating and should seal by themselves. If not, then they're probably burred or ridged and if you seal the thread to stop this, it'll just leak round the pipe instead !
After changing the fuel taps a number of times, and still having problems, I decided to fit some slightly more modern ones, unfortunately I wasn't confident that the adaptor needed to go into the tank had enough 'land' on it to seal with the washer, so I bought some of this stuff and can highly recommend it.
