Lying flat should neck veins normally be flat or destended?

It depends : There are multiple factors that go into neck veins pressure.Your position whether you are lying flat , the position of the bed you are lying on whether it is flat, whether you are relaxed , if you bear down or valsalva that will distend your neck veins. Other medical factors influence this as well, but yes your vein should be flat if all of the other things are optimized to keep them flat.

Flat: Neck veins distend when we lie with our head lower than our heart or when we strain. With straining, the pressure within the chest increases and the blood flow through the neck veins decrease resulting in them enlarging. This is normal. Levels of hydration, genetics, outside temperature and any increase in pressure within the chest will enlarge the neck veins. Neck veins collapse with standing.