The 16" wheels fitted to the 12 valve models use the conical nuts to centralise and clamp the wheel to the hub. The later wheels as fitted to the 24 valve models, use the later style nuts that have the washer. The wheel is centralised by the protruding section of the nut that locates into the holes, but the clamping force that holds the wheel to the hub, is transmitted by the washer contacting with the flat surface on the rim.
Just to confuse matters, and if I remember correctly, the 24 valve washer type nuts do have the conical end on them. This could be to allow them to mount a spare steel wheel, which requires the conical nut to locate and clamp it into place.
The 12/24 valve distinction is just my working knowledge on the issue, and shouldn't be regarded as fact. I'm sure someone else might be able to shed some more info on the specifics.
If you mix the different type alloys and nuts, the alloy wheel will not be properly clamped and centralised onto the hub, and there is a strong risk of losing a wheel.