Power Valves are held shut by vacuum.
When the Vacuum drops below the rated value of the Power Valve ie: 3.5", 6.5" etc. the Power Valve will open and stay open until the vacuum signal is above the rated value.
Typical Drag Race cams have a ton of overlap causing a low Vacuum Signal to begin with thats why a lower rated PV is used.
During the Burnout and Staging the Power Valve will likely remain shut and the Carb will only be receiving fuel from the Main Jets which are smaller ( than when run w/o a PV ) so it will be running leaner.
The other guys are absolutley correct in saying that running without them is one less that can go wrong. I was just pointing out that they can be used as you asked for both sides.
I will always run them in the Primary Side regardless. The Secondairy's I might try one ... I might not.
A PV will not affect Idle ( unless it is damaged, ruptered by backfire ) regardless of the rated value as your not pulling fuel from the boosters at idle.