When building a house, it is actually quite the opposite and most floor joists and floor trusses are laid perpendicular to each other. One reason for this is to lessen the load on the wall. If the joists and trusses are running in the same direction, this means that they will all be attached to the same wall, increasing the load that the wall is carrying. When you attach them perpendicular to each other, you will be distributing the load evenly amongst the other walls, making your floor more secure and sturdy. However, it is also possible to line up the floor joists and floor trusses, depending on the design of the house that you are building and its structural limitations.