Does the initial position of the B-tension screw matter?
It doesn't matter how it's set when you first begin the installation. If it were out so far that it prevented the guide pulley from clearing the large cog at all, that would be a problem, but you'll see that pretty quickly. The ideal time to set it is after the wheel, derailer, and chain are all in place, after the limit screws have been set, and after the cable has been anchored and the tension is in the ballpark of correct, but before final adjustment of the cable tension, since it will affect how smoothly the system shifts in each gear and you want to see an accurate representation of that as you decide whether to tighten, loosen, or leave alone your barrel adjuster.

Note that we now live in a world of wildly different recommended pulley/cog gaps, so if it's a newer derailer and especially if it's a 1x drivetrain, it's a good idea to check what the manufacturer says it's supposed to be.