I put a very thin layer on the threads of both.
And once assembled, if there is a leak from the union, I put a super thin layer of teflon dope in there also.
But they usually don’t leak.
Some people will probably say never to add any dope inside the union, but hey.
Putting some valve grease on the big threads of the union fitting helps it to tighten extra tight.
And helps to get it apart more easily someday.