I actually think that when they mow the grass in perpendicular lines, they do 6-yard lines. That way the third line is at the 18-yard line (for the 18-yard box). Also, the lines help linesmen judge offsides better. I have recently seen pitches mowed in a circular pattern, and that seems just to be showy, and i think they may stop that in order to help the refs with the offside.