There was a discussion about this point in another forum. A guy remembers that his boss (auto garage) telling him to always prefill. I think the reason was a solution to head-off a possibility of a human mistake.
If you don't prefill you have to wait until the reservoir fills. Wait for the oil to travel through the 1/16" hole in the tube and down the hose back into the engine. It shouldn't make any difference in the oil pressure and bearing lubrication at idle speed. Assuming you first filled with 5-1/2 quarts and not 6 quarts to avoid overfilling. Stop the engine and add oil to the full mark. Run the engine and check again. Maybe repeat again several times. Prefilling can save time and maybe prevent under and over filling.
On modern engines with full flow oil filtration, the oil circulates in what seems like seconds.
