We feed all live and have had no problems. Like stated above if they dont eat it well, it's not a waste. With F/T if they dont eat it's wasted. We have to many snakes for them to not eat and have to throw away a bunch of rats.
As for the never re freezing them. Once they are thawed and warmed and left for awhile they will start to harbor bacteria. Just like with humans you dont thaw out a stake and the refreeze it.