How important is the first guide size with braid?

[Sweetwater]

You need to make sure that the first guide doesn't choke the line coming through. Whether you need a 50 or a 40 depends completely on the size of the spool and the distance of the reel spool from the first guide.

Make a few casts with just an oz or two of weight and watch the line going through the guide, if it's hitting the sides (and you might even hear it), re-tie a 50.