There were times when I needed a bunch of skinny quarters and I wished they had torn the fabric to get the straight of grain. I really needed to make 2 1/2" strips width of fabric, but the elbows were so bad, I could only get the strips half-width, which meant they were on the bias, which isn't good.
With good fabric, tearing only distorts the edge. It helps you with folding the fabric and you can always trim off the little bit that's distorted.
Panels are notorious for being wonky. I used one recently. It had cute little kittens that, at a glance, were squares, but when I measured, they were 1/8" taller than they were wide. I wound up choosing the shoe-fly pattern with the kitten block in the center. Made my side pieces the same size as the sides of the kitten blocks.
You just need to make your own judgement call on when it's right to tear (when you really need the straight of grain) and when it's better to cut (like when there's a directional pattern on the fabric).
Happy quilting!
About the safety of using those razor blades. I never cut myself on them even though I was just a kid. The only time I have cut myself is with my rotary cutter. That's when I realized that I was a quilter because all I cared about was not getting blood on the fabric!