For simple walls I generally use boxes. Deleting the backfaces of walls, which are very simple meshes is, in my eyes, both a waste of time and opening a door for lots of issues harder to solve. Most of the times walls will be completly hidden and hence culled out.
In short, I recomend you to keep walls with all of their sides to be sure lightning will work correctly. If this is for a mobile game and you will use baked lights be sure to enable precomputed visibility to further improve performance.
In current gent games this kind of optimization is deprecated since the difference you will obtaing in performance is almost null (even more if the lights are precomputed).
Another piece of advise is: Do not focus on optimization in the early stages of your game. Get it running, make your level, optimize later if and only if you need to.
Hope it helps. Make it a great day!