It should be completely full. Ideally, without any air pockets although that's probably not easy to achieve.
If the air pocket is too large there might be a possibility that the ambient chamber won't see the full ambient pressure. In a full chamber, lube is not compressible, so the water can exert pressure through the trim ring on the lube, and the chamber will see the ambient pressure. If there is an air pocket, air is compressible, so for the pressure to be the same, something(water) will have to move in through the holes. Unfortunately the trim ring hinders this, so the insides of the ambient chamber may end up being at a lower pressure than ambient. The symptom will be the regulator becoming very hard to breath at depth, and being normal at shallow depths. I believe this once happened to someone with an Atomic regulator.
But then I've only read one instance of this, so as long as it's full enough (meaning some lube oozes out the holes when it's first pressurized) it should be fine.