Grabbing your penis 1 inch before the glans causes some blood to be collected in it, which I think is good while stretching and it should be ok as long as you don’t put too much pressure. However, if you grab your penis right before the glans and stretch, you don’t have much blood in the glans and some blood might even return to shaft from squeezing the glans which I think is bad and might cause injury. I had a minor injury when I stretched hard by grabbing right before my glans and I had my glans bleed couple of times when I put too much pressure on glans filled with some blood. So stretching has potential possibilities for injuries if done wrong ways and one should listen to his penis response and go easy at it. My two cents and I’m no expert in stretching.