The main reason porch boards run perpendicular to the house is to eliminate butt joints.
There is no reason you can't run the boards parallel to the structure, and some people prefer the way that looks.
One possible benefit to running the boards perpendicular is to aid water run-off...but I don't think it matters very much with today's wood preservation products. My own 30' wide by 10' deep porch, which was built in 1908 using 3" tongue & groove pine, was installed perpendicular to the home had only a few areas that needed replacing after 100 years. It has about 1/8" to the foot of slope away from the structure.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright