Do we need Dielctric union between copper line and stainless steel valve?

308 or 316 SS would not. Most valves are that alloy. The weak link is black iron and you don't have that.

I agree with ma. No need for dielectric unions on brass or copper to stainless. However, it really all comes down to what you are doing. Say you are installing a tankless water heater, and the install instructions call for a dielectric union. In this case you will want to install the specific type of union they call for to keep the warranty intact. Other than that, no worries.