Actually these are questions best asked of your LAQ. Many LAQ do not like seam running the length of the quilt because they get uneven buildup. Others don't care one way or the other. My favorite is to use wide back fabric. In the long run it is less expensive then 42" wide fabric. Howeve if you are working with a quilt as wide as 108" which is HUGE you would need to enlarge even wide back as most LAQ require a minimum of 8" larger on sides (4" on each side).
If I were dealing with only a 45" wide quilt I would use the diagonal method.
http://www.pinkchalkstudio.com/blog/...-flynn-method/
If I don't do a wide back I will often piece the back with orphan blocks and have many seams running across the back but I do my own LAQ so I only have to worry about pleasing myself. I know of many LAQ who dread getting a pieced back and some outright refuse.
Here is a link that to a post that Jan in Va did some wonderful diagrams for piecing quilt backs.
http://www.quiltingboard.com/main-f1...g-t223319.html
Ultimately though, you should ask you LAQ what they prefer.