Mounting widthwise looks cool, but practically speaking, mounting lengthwise makes it quicker/easier because you can access all of the pins to deploy or stow the boards from one side of the vehicle.
^ this. I ran mine across for a while and it looked better but was a pain. My maxtrax are slightly tweaked and I have to wiggle them off the pins. Picture 5-foot nothing me climbing up each side of my truck 3 or 4 times trying to get them off the pins .