It's not necessary per say, but if you block it, you have to jump roughly 6 jet sizes in the rear to compensate for it.
I've run mine with and without. Quite frankly I like mine with the rear power valve in place. The throttle transition from primary to secondary is much smoother, especially when you whack it, and I liked the AFR a little better with the PV while at the same time not having such a big spread in jet size from primary to secondary.
With the wide band, and rear power valve removed, and jetted 6 sizes up on the rear, my WOT AFR was 12.9 Being at 5,000 ft that's a bit too lean for my liking considering that number jumps a full point at sea level. When I went back to stock rear jetting and reinstalled the 6.5 power valve, the WOT was hovering around 12.5 So it would appear that removing the PV and jumping the rear jets 6 sizes really wasn't quite enough to give the same results.