Roof Sheathing Joins
You are correct in that a 1-5/8" 2x4, less the 1/8" expansion gap, leaves only 3/4" for nailing. However, if you use the correct sheathing nail, and mark a line 3/8" off the edge of the sheathing, you shouldn't have a problem if you nail straight down. Even if you split the occasional 2x4, it will not be a structural problem, possibly cosmetic, but you are building a shed, not a piece of furniture, so don't sweat it. By the way, if the rafters are more than 16" oc, you are supposed to use a sheathing spacer plate at the center of the sheathing to maintain correct spacing. If you use conventional 16" oc, no such spacer is required.