Each dyno is going to be different so take that into consideration when you get your final numbers.
Why dyno with the hood closed? I always do to make sure its a more accurate reading. If you are only concerned about bragging rights on a dyno plot, then yeah, you can dyno with your hood open. On the street, air flows form the front and bottom of the car. If the dyno facility is properly equiped, they will have a fan specially made for dyno sessions, not just a box fan that you buy from home depot or something.
People dyno with the hood open because they want high numbers or they dont have the right fan. This happens a lot when an intake manufacturer wants to show a gain with their intake systems. Dyno with the hood closed stock, slap on the intake and dyno with the hood open and a fan blowing on the induction filter.