Originally wood floors ran the length of the room or at a 45 degree angle based on the fact that they were the only thing that connected the floor joist that run the width of the room.
Then they came up with 1x6 floor sheathing to accomplish this structural task. At this point the hard wood flooring material would then cross the sheathing at a perpendicular angle and be parallel to the joist.
Now most houses have plywood sheathing therefor the floor material can be installed in whatever way that is aesthetically pleasing.
Here is a rule of thumb breakdown;
1:Wide boards run the length of the room to replicate Old World Construction.
2:Narrow Hardwood boards run the width of the room to replicate Turn of the Century to mid 1900's Construction.
3:Any variation is acceptable as long as it is aesthetically pleasing.