if you want to spund early to capture some co2 for a natural carbonation i strongly suggest using a spunding device. They can be set for the desired preassure and will release exess co2. prv's shouldnt be used in this way, they are an emergency device.
On the other hand, I belive in getting rid of all gas from primary fermentation, letting all those So/so2 gasses to wander out of the beer. Idealy for natural cabonation I would use a karusening technique and prefferably in a secondary tank. No matter how often you pull yeast some will remain and you have the chance for autolysis flavors with extended conditioning.
Brewmaster, Fitger's Brewhouse
"Your results may vary"