A small nipple size dot at the center of the CPU heatsink spread out vertically and horizontally with a credit card has ALWAYS created a nice smooth even coat across the entire surface of the heatsink for me over the years.
It may LOOK smooth but if you use a glass plate to simulate the heatsink or CPU being attached to it and scan the result with a microscope, you will likely find thousands of microscopic air bubbles between the glass and paste caused by fine streaks from the card's edge. With the center pea method, air gets displaced out as the pea flattens.
The "credit card" method is useful for coating unusually rough surfaces. For relatively smooth surfaces, it most likely creates more bubbles than it prevents.