We find it's often difficult to get the cabinets and the bulkhead, or soffit, to align, so it's better to break that plane and give the cabinets a surface to finish to by extending the soffit out. It also creates a shadow line that's nice. There is not hard and fast rule about this, but unless you can have absolute precision between the different materials (cabinets and soffit) it's best to not have them align.