The quilting teachers I've had said the same thing as Tartan -- you lose around 1" in damaged fabric on each side when it's torn. You need to cut that off before you can start cutting your pieces or it can impact your precision in addition to generally just not looking pretty.
There are times where I will rip ... if it's extra wide fabric or if it's more off-grain than usual (within reason, some fabric just can't be straightened). It is the most fool-proof way to get a straight edge, but it does come at a price. I typically prefer shops that rotary cut my fabric. It tends to be the most accurate & really isn't that difficult to for me to straighten at home with as little as an extra 1/8yd of fabric.
There is one LQS by me that I have to really be careful when I shop there. They sell amazing repro fabrics, but tear everything, including fat eighth's & I never know what I'm going to get unless I open everything up and measure it in the store. It's really not ideal to tear on both the lengthwise and crosswise grains. They claim it's extra accurate, but that's certainly not been my experience. They routinely vary by 1.25 - 1.875" and the worst one I had was somehow on the bias. They claim it was rotary cut but it was awfully frayed & was 7.5" long on the right side & 11" long on the left side -- the width was better but still varied by over 1" and then on top of that, I had to cut off the pulled/puckered threads and square the thing up. I stopped shopping there for a long while, but they were very apologetic when I finally returned and always offer to help me check to make sure I get the "right" pre-cuts for me. I normally only buy manufacturer pre-cuts or yardage, but this one shop gets special deals on random repro fabrics that arrive in all sorts of lengths & then get torn up by store employees to form FQ's or F8's. So many fun & unique little prints that are all far too tempting! Just have to be cautious & know who is cutting or tearing your fabrics so you're prepared for the aftermath.