So I have integrated sensors into furniture a few times - couches, chairs, tables, etc. Short answer to your question is that for prototyping it depends on what all you need integrated, and if you are building the furniture custom. Most of the time it is much easier to install the electronics while building the furniture as a one off, than it is to retrofit into an existing piece.
For production though, your problem is that most people who can build you furniture in quantity are not the same set of people who can work with electronics. So you have to design both the furniture and electronics to be installed, much like with a car, in more standard components that abstract the electronics into modular components.
Can you give any more information about what you need?