ORIGINAL: da Rock
If you're going to finger choke the carb, no need to open or close the throttle. You're blocking off all atmospheric pressure with the thumb and that makes the throttle position a moot point. But to add some positive advice.........
Actually there is a good reason. While you're closing off all atmosheric air, you're also closing off the needle valve. The engine will prime quicker (3-4 flips) with the throttle fully open than it will with the throttle closed. I've had times when trying to prime and forgot to open the throttle, and no fuel moved at all into the fuel lines.
If you're using an electric starter, there is no reason to prime at all, unless you want to make sure you have some lubrication in the cylinder before hitting it with the starter.
Brad