I have to use a frame and computerized quilting system to quilt because of a few health problems. I have no idea what to do when quilting at a table on a DSM or a mid arm sit down machine. My quilting is not show quality, but it impresses my kids and grandkids.
I've heard it both ways about sewing strips of fabric on the straight of grain (perpendicular) making the backing stronger than the sewing them parallel. IMHO, the seams are always a weak point, so accurate seams are the main thing to worry about.
I do not rip fabric to get straight of grain. Just so happens that even LQS quality fabric can be printed off grain, so when you tear the fabric you can loose more fabric than you thought you would - and it still doesn't look straight.
If you purchase fabric locally, it's usually easy to see that the print of the fabric is off; it's not so easy when you buy on line.
So what I am going to say assumes:
1) that you have a frame to mount your quilt on or it will be sent to a long arm quilter:
2) You need about 10 - 11 + yards for the backing. That does not include binding.
3) a minimum of 40" WOF
after selvages are removed.
What to do:
* Minimum to get 108" width of back is a 3 yard strip. (3 x 36" = 108").
* You will need 3 rows to sew together to get a back length greater than or equal to 108 inches.
* To add 8" more width for attaching the quilt backing to a frame, you need a
minimum of 3.25 yards per strip. I would make it 3.5 yards to account for any straightening you need to do before or after you sew the backing together.
++Note: Alternatively, you could cut 2 separate 4 inch strips = to the length of the quilt +8 inches and attach each of them to the sides of the backing to get the extra 8 inch width recommended for quilting on a frame. This is useful when you discover you don't quite have enough fabric for the backing.
* Each of the 3 rows of 3 yards sewn horizontally (or vertically) has a WOF of 40+ inches.
* The 3 rows sewn together will equal 120 - 1 (for two seams: 1/4" x 4 = 1) = 119". (If your wof after removing selvages is more than 40', the length of the 3 pieces sewn together will be greater.)
* You need
108" + 8" or 116" for the length of the backing to attach the quilt to the frame. Measure and trim excess
* Two ways to M
ount the quilt back on the frame
1) My preference: Attach the pieced backing on the frame so that the seams are parallel to the take up bar.
The extra two layers of fabric in the seam are then sewn evenly across the fabric, producing a smooth row of quilting. There is a slight bump across the quilt, but it is the same all the way across the quilt and will not affect quilting subsequent rows.
2) Attach the quilt so that the seams are perpendicular to the quilt top. When you roll the take up bar for the next row, a bump will appear where the extra layers of fabric are quilted. Since you will have two perpendicular seams, you wind up with two bumps. After a few rows, the bumps are obvious and they throw your quilting out of alignment. That makes it difficult to maintain your spacing for the quilt, which results in crooked quilted rows, quilting overlap where there should be none and gaps in quilting where there shouldn't be. At least that's what happens to me! I'm sure that many LAQ can do this and not have the bumps or are adept at working it into the quilting.