One thing you might check is to make sure under the Renderer > Default Quality Rendering is checked, the Renderer is located in the panel bar at the top of each view port.

If you have it set to High Quality Rendering then it will always be viewed as a HQ render and negate the "visuals" of the Paint Skin Weights Tool.

How are you selecting the joint to paint ? it's Important to select them from the list of influences in the paint skin weights tool settings.