Tearing fabric to obtain straight of grain?
 

I am not a fan of tearing fabric for straight of grain when quilting, but I do have a question on how it should be done properly.
I was recently at a quilt shop I had patronized many times previously where they cut the fabric with a rotary cutter. This most recent visit, the staff member measured the fabric, placed her finger at the correct mark, snipped the fabric (on the fold) and proceeded to tear it to the selvedge edges, gripping both layers of fabric at the same time.
She was so quick, and I was so surprised, as previously this store cut the fabric. I do not like to buy fabric where it is torn. (Personal preference of mine)
Subsequent to this, when I went to use the fabric, I had nothing but trouble - ripples, shifting when trying to cut/pin etc and it didn't seem to lay flat as easily as usual.
When you plan to rip fabric, I was under the impression that one snipped the selvedge, tore to the opposite side and then cut that selvedge.
What is the correct way to do it? My most recent experience seemed to be really detrimental to the fabric.

Haven't seen fabric torn in recent years. Once it was common for woven fabrics, I always saw it snipped at the selvedge, then torn. Some places, JC Penney comes to mind, had a machine they would run the fabric thru to measure, push a lever to make a cut, then tear the fabric.
That was also in the day when you were not allowed to get your own patterns out of the drawer. You had to ask a clerk for them. Always a problem for me-I learned to sew at a young age and loved to look thru pattern books. Clerks didn't want to help me cause I was "too young" to sew. Got in trouble more than once cause I helped myself!

Ripping fabric
 

I did this once yrs ago so I can’t remember how much yardage. Always heard it was the straight way. I ruined the whole darn piece because it ripped at such an angle. Couldn’t use it for its intended project. Never did that again.

I tear my own and my favorite shop does, too; they rip from the selvedge and I usually rip from the fold. Both ways work.

If you don't like your fabric torn just mention it when you take it to the table. Even if they weren't going to tear it anyway it's good to be clear upfront.

Ripping fabric on the crossgrain (selvedge to selvedge) does considerably more damage to the ripped edges than ripping fabric on the lengthwise grain (parallel to the selvedge). This is why crosswise rips result in much more rippling of the edges than lengthwise rips. One can accurately assess the depth of the damage from the rip only under a microscope, as most of the damage is not visible to the naked eye. Under a microscope, the damage to the fibers becomes more visible.

Exactly how much damage is done to a fabric ripped crosswise probably has more to do with the individual fabric's weave, tightness of weave, thread count, etc. than with whether the fabric was ripped in a single layer or a double layer. However, ripping a double layer is probably more rough on the fabric than ripping a single layer.

In my opinion, there is no correct way to rip fabric on the crosswise grain if the fabric is to be used for quilting. This is because crosswise ripping results in an unnecessary loss of fabric integrity. Because we want our quilts to last a long time, integrity of the fabric fibers is important. No one wants some patches in a quilt (or a joining seam for the backing) to give out earlier than the rest of the quilt. Straight-of-grain is much less important in quilting than fabric integrity.

There are instances when ripping fabric is fine -- when making rag rugs, for example. For garment sewing, though, when finding the straight-of-grain can be important, I was taught to pull a thread and cut. Ripping is simply not as good a method. The only positive thing to say about ripping is that it is fast.