That's what is best, but a centered pattern does nothing for valvetrain stability. It just gives the largest amount of contact area between the roller and the valve tip. With minimal sweep, the valvetrain will be stable, even if the pattern is off center. That's why the narrow pattern is of primary importance, and the location of the pattern is secondary. If it's centered, great! If not, it isn't going to cause a problem unless it's off a good bit, or if titanium valves and/or very small stems are utilized.