I've never heard of such a thing. Feeding live is fine and safe when it is done responsible (IE: don't leave the mouse/rat in there with the snake by it's self for hours, days, weeks, etc...) You sit and watch and make sure nothing goes wrong! Feeding f/t is fine too. I say whatever works, go with it. I have snakes who eat f/t and others who will only eat live. All of my snakes have nice muscle tone. I know at my job we feed a few thousand boa's f/t rats weekly. All of the boa's have a nice muscle tone and aren't "mushy" at all. All of our ball pythons (few thousand) get fed live, and they too all have nice muscle tone.

EDIT:
Dot you are very right. Some will grab and constrict the f/t rat/mouse... while others will just grab and start swallowing right away.