First, you'll almost never see a valve block to be complete "off" in a model. You'll always see a minimum area or flow fraction. This is to avoid a sudden change to the systems of equations in the middle of a simulation, so there is some good numerical reason behind this. Also, physically, a valve is never fully "off". Leakage is real. As long as the leakage is a neglible amount, you may consider the valve off.
Now, to your question. The Orifice (2P) block from the Simscape Fluids library is the right one to use. Its equations are a bit more robust than the ones in the Local Restriction block. I tends to get more stable results from Orifice (2P).
