Determining length and width can be important. I recently helped my mother do the cutting on a rather complicated pattern where triangles kind of spun out of a not quite octagonal center to form a star. Each template for the triangles clearly stated how the lengthwise grain should go. I am glad we cut it as they stated, because that pattern was extremely challenging the way it was.
Also, I like to have my borders on the lengthwise grain when possible.
Dayle