OK, this is some hardcore fluid dynamics. Hang on tight. But understanding this concept is key to understanding how seams may effect laminar/turbulent flow and boundary layer separation.
As a baseball moves through the air, the pressure on the front of the ball is higher than the surrounding air, which is intuitive. BUT, as the air moves around the ball, the air velocity increases and the pressure goes down (due to the Bernoulli effect), actually becoming less than the surrounding air near the hemisphere of the ball (dashed line).
Now, for the hard-core fluid dynamics. Disturbances in the flow, such as a baseball seam, can cause the flow to transition from laminar to turbulent, or can cause the boundary layer to separate from the surface. The flow does not care much about the pressure itself, but it is very sensitive to how the pressure is changing, which we call a gradient. The pressure is decreasing on the front of the ball, and we call this a Favorable Gradient. In a favorable gradient, the flow is stable, meaning that if you perturb it, say with a baseball seam, everything will return to the way it was. In other words, seams on the front of the ball will not have much effect. This becomes less true as you approach the hemisphere of the ball, indicated with the dashed line. The favorable gradient is strongest at the front of the ball and vanishes at the hemisphere.
Past the hemisphere, the pressure is increasing up to the point of boundary layer separation. The flow is very unstable to disturbances (like baseball seams) in this region. BUT, once the boundary layer is separated, the seams does not matter at all.
Bottom line, seams only matter in the region near the hemisphere. An example is shown below. The seam on the lower front of the ball has had no effect. But the seam just past the hemisphere on the top has caused the boundary layer to separate.
