Do I sanitize a ball valve after taking samples?

As with all ball valves, you should sanitize the output side before running off or attaching your racking tube.

If you are taking samples from the ball valve (as you should) after each sample you should spray an ample amount of StarSan into the closed valve.

This will rinse away any sugars which could offer a source of food for bacteria.

If you mix StarSan per the instructions the Ph of the solution will be well beyond levels that bacteria can survive in.

Another sense of security you could perform is to put a Tap Plug or Cap into the hose barb of the valve after you spray it with StarSan. This will ensure an extended contact time for maximum protection.