If the derailleur's upper pulley is bumping the largest cog, it can definitely have an affect. By screwing it in, you can change the derailleur's angle so you can make the pulley stop bumping into the cog. For the most responsive shifting (in theory, anyway), you should have the screw unscrewed as much as you can without allowing the pulley to hit the largest cog. Small gap between the cog and the pulley, in other words. I have certainly had to tighten the screw to make the pulley stop hitting the cog. It's a simple adjustment that's there if you need it. And again, for optimum setup, leave as small a gap as possible between pulley and cog.