You dont need to wait till soft to pick a sugar apple...as a matter of fact I personally dont recommend it (the rish if insexts or vermin getting to it or the splat factor is great).
Whether red or green type, it will lighten in color, especially around the sections, and the sextions will seperate. If picked properly and hard, will soften up in a day or two and be perfect eating (exxept for the high seed count).