I always pack them Mick, as per here:
However, it's a good question and something I've noticed myself. Where 'bearing savers/buddies' are fitted the front bearings will always be stacked with grease, but the rear ones not so much.
Personally (and it's all subjective and up to the individual with these things), I don't leave much of an air gap at all. The bearing savers (not the type which are just glorified dust caps with a grease nipple in them) are sprung loaded and put continuos pressure on the grease. They also have a small hole, in the side of them, so when the grease reservoir is full and under pressure and excess comes out