Usually plywood is an odd number of plies. Sometimes the outer plies are much thinner than the inner plies, so it's going to vary according to how the plywood is constructed.
So, let's say your plywood is three plies. That means only the center ply is running with the grain. If yours is five plies then two of the five plies are running with the grain.
In thicker sheets it matters less because the plies are thicker and there are more of them. In thinner sheets, the odds are stacked against you so I'd say yes, it's weaker than if the grain were running with the length.