No. You can use InHouse directly, regardless of where you live (within the U.S.) — just sign up and twin your space for $49.
The difference if your building is a partner:
Your building may provide your unit's twin as an amenity (so you skip the $49).
You may get a smoother setup since your unit's floor plan is already on file.
Either way you shop and buy furniture at member pricing. Not sure if your building participates? See "My building offers InHouse — how does that work?"