I think it is good to work out the terms. It avoids so much confusion when people are saying something and it sounds like something else.
I commonly hear poly/mono couple to imply that one of the people is open to outside relationships while the others is not looking nor desires one. It doesn't mean that the poly one is really dating someone else and it doesn't mean that the mono person will never date someone outside the relationship.
At least that is the way I have heard it portrayed.