Conventional wisdom on hardwood flooring is very straightforward: always run the boards parallel to the longest side of the room. But there's an exception to every rule, and some lessons to be learned before it's time to install.
Floor joists must must be perpendicular to the direction of your hardwood floor for added strength (unless you have a super duper substrate). If you don't lay your floors this way, you may void your flooring warranty because sagging is more likely to occur. It also makes sense to orient floors toward your external light source, minimizing the visibility of imperfections, and in a vertical direction from a grand front entryway.
All of this advice goes out the window if your client is afraid that her very large, long room will resemble a bowling alley with length-wise planks. In a case like this, it may make sense to run the floor across the shorter sides of the room. It is also important to check in on the installation to make sure the layout is aesthetically pleasing. Cuts should be in random spots with no detectable sequence unless you are trying to create a pattern. It is also a good idea to check the variation of color in a natural product such as hardwood. You'll want to spread the lighter and darker pieces evenly around the space.