It wont make a difference to your valves, it's the power curve that gets the real effect from tube size.
Small pipes make more torque in the low range, but restrict the top end flow. Big pipes are better for high end power, but comes at a loss for the low end torque.
The smaller pipe keeps the gas velocities up creating a vaccum that assists in scavenging exhaust gasses from the combustion chamber, making for a cleaner burn on the next cycle.
Larger pipes can have issues with back flow but that's where headers make a difference by keeping the gasses moving in one direction with a directional collector where typical stock manifolds get more interference without the smaller pipes to keep things moving. Once you open it up & get it turning high RPMs it's all about how well she can breathe.
Bottom line: It's up to personal preference when it comes to the exhaust and what size tubing you use. Dual 3" pipes on a V8 that doesn't see race duty have no advantage other than they sound better.
I'd go with what 50Deluxe says, try em straight, then if it's too much noise for you (or the local police) add on some mufflers.
