Is it better to have tapered or straight pipe threads on a rotary joint?

Actually you are asking about the terms "non-sealing pipe threads (tapered threads)" and "sealing pipe threads (column threads)", which have their own advantages and disadvantages:

Tapered threads are good for neutrality and can be added to improve sealing performance of hemp or raw matting tape, mostly used in civilian applications. It is mostly used for civil applications.

Pillar thread is the use of internal and external threads between the extrusion to achieve the purpose of sealing, the advantage of higher strength, the disadvantage is that the manufacturing precision requirements are high, mostly used in petroleum, chemical industry, military occasions.

But in recent years, the use of taper thread fastening, and more thread end sealing, it is absorbed the advantages of the above two.