From my very limited experiences...
I met Marty's wife the very first time he and I saw each other in person. And he met my husband at the same gathering. It was important to me, as a poly-newbie, to make sure via direct contact that his wife was okay with us dating, and to give Marty the opportunity to see that my husband was okay with it. The whole "trust, but verify" thing.
On the flip side, my husband had a relationship for a couple months this year that he hid from me, and since the other woman did not have a "oh, I should meet his wife to make sure things are on the up and up", when I did find out about it, the poop hit the fan. He had told her things were okay, so she had no idea I was in the dark.
Just my thoughts. I'm not saying anyone in your situation is dishonest, but I am a huge fan of the brief face to face to get direct confirmation from the metamour that things are truly okay with them/they are in the loop/etc. And I'm not talking just about intercourse/other physical things. I'm talking about any relationship - casual dating, casual sex, serious dating, whatever it is. I would much rather have a brief awkward conversation than find out months later the other partner is not actually cool with it.
"This, too, is sacred."
I am my own primary.