Hey Richard,
You are doing a fantastic learning curve, congrats!!!
About add a extra wall, its really important.. otherwise, you will have less photons flying in your interior scene, which can make appear artifacts (blotches), requiring more photons and slowing your render.. so, a simple wall, make your render beautifull with a faster render time, the second side Pro about a extra wall is the fact you will not have flicking light if you do animation.
Tip.. look more interior real photos to match your 3d rendering.. you will learn a lot reproducing them.
Congrats Again..
Thanksss
Tom