If there's any question at all, you should check with your local inspection authority. I'd say the requirements would be the same as for other pressure piping applications - registered welding procedure, licenced welder, and all exposed threads covered by the weld.
It is OK and a good idea for threaded pipe conveying any hazardous or dangerous fluid and gasses.
ASME B31.1 (1995) both permits and encourages what is called "seal welding" (See section 111.5 and 127.4.5)
Bear in mind that you have probably ruined the fitting for any possibility of dis- assembly.
Oh, and a thought process should also take place..... Why have threaded fittings in that system in the first place ? Why not make the entire system socket-welded ?
