Nail some 16D nails on the rafters between the sheets when installing them. This should give enough space between them. You don't want them tight. Make sure to remove the nails before you trip on them. The sheathing should not overhang the roof at any point. Have it flush or close to flush to the edge and gable end fly rafters. Use style D roof edge before installing any roofing, unless you are having a wood fascia.