Prism99

I don't like tearing fabric. It leaves microscopic damage up to 2 inches from the edge of the tear. This is especially true when tearing cross-grain (across the width of the fabric), as would be done in a fabric shop. Tearing along the straight-grain does less damage to the fabric.

For quilting purposes, it is not critical to always be exactly on-grain to the thread. It's more important to be close to on-grain for piecing. For example, you want the edges of your quilt top to be on-grain so they don't stretch out of shape from handling during sandwiching, quilting and binding. Bias edges must be handled very carefully to avoid distortions. For cutting out strips and pieces, it's enough to be close to on-grain; it doesn't have to be perfect!

With garments, grain is extremely important in order for fabrics to drape properly over the body. Quilt tops that are made out of many small pieces of fabric are going to have grain lines going every which way. Keeping strips and pieces cut close to grainlines helps ensure piecing accuracy so all pieces fit together in the end.

Edit: I would add that most quilters do not tear fabric to determine straight-of-grain. It's enough to "eyeball" it by lining up selvedges. Where we need to be very precise is with the cutting angles. For example, once a fabric is folded, a cut needs to be exactly 90 degrees from the fold (not the selvedges!) in order to have a straight cut when the fabric is unfolded. The cause of the dreaded "V" cut in strips has nothing to do with grainline, but everything to do with whether the ruler was positioned exactly 90 degrees from the fold line.