Just got most of my parts for my build. The left shifter is double/triple compatible. There is a tag in the left shifter that says to set it up for a double by using the first two positions (granny and middle positions if it was a triple) and then warns that attempting to shift in the cable pull direction when you are already on the big chainring (middle position of the shifter) will break the shifter. If this is true, then why even set it up that way? Why not use the top two positions? Then accidentally shifting down when you are already on the small chainring will just let a bit of slack out in the cable. (The Sora on my current bike is set up this way) The tech doc says there are trim positions in all three spots so that can't be the reason. Am I missing something?

I think it's a leftover recommendation from previous Shimano brifters which only had trim positions in the first or first two chainring positions (like Sora and Ultegra 6500 respectively). With trim for all three chainrings, there's no reason not to set up your double using the last two other than the slack cable issue. That would bug me but if you don't mind, go for it.

Thanks. If I only shift normally it won't be an issue either way but slack in the cable will bug me a lot less than a broken $150 shifter.