If you intend to use wood chips or chunks, the vent needs to be completely open. If you have the vent partially closed, the chips can be sitting there starved for oxygen not smoking, but lit. When you pull the side chip loader combustion can take place when oxygen rushes in and you will hear a whoosh. It may be very rare but if there is grease dripping down from your meat products cooking, it is possible that the grease could ignite. (This was a problem for MES when they first hit the market and Masterbuilt had to recall those early units). It can still happen if you don't leave the vent open.
So for proper combustion of the wood vent wide open. As to cooking without wood, should the vent be open, that is a matter of opinion.