Yes that's right. once you define the vertices for the front and back (near and far) faces you have given all the information needed to construct the cube. Anything else is redundant and waste memory.
The only thing you need to be careful about is this : When you actually use the vertices to construct the cube order matters.
By default opengl expects for the vertices to be provided counter clockwise. I believe you can override that behavior, but I have spent many hours trying to figure out what I thought were coding mistakes that turned out to be vertex ordering mistakes. As long as you remember to always construct the vertices in the counter clockwise manner (as viewed from the outside of the cube) the rendering should work fine.