First of all, I'm pretty sure you are talking about blow off valves, not values. If not, what the heck is a blow off value? A great deal at the dollar store?

Secondly, there is absolutely no value whatsoever, and may even decrease performance, and harm O2 sensors. Why? Because our cars use MAF systems that work on metered air. If you allow metered air out of the system before it is combusted, the ECU will not be aware of it, and will inject more fuel than is required, which will result in running very rich for short periods, or whenever the valve opens. And that unburnt fuel will contaminate your O2.

If you really want a BOV, go with a stand alone engine management program.