I reccomend reading up on internal parasites, especially round-worms. I have never found someone to continue to use live after doing so, unless of course the snake will NOT eat f/t.
Live rodents, includeing pre-killed, and those that were frozen for less than two weeks can pose a health concern for your snake.
The majority of rodents available will have some form of parasites in their gut. When you feed them to your snake, your snake can contract the parasites.
Round-worms are the most common, and often don't pose an immediate problem to your pet.
The problem arrises when the worms stress out your pet's organs or allow other parasites that naturally exist in your snake's gut to reach large proportions in number.
Round-worm infestations will usually prevent females from being able to breed. I am not sure why.
My veterinarian is the vet for the Pittsburgh Zoo, and he was the one who pointed out problems of round-worms to me. I had a three year old female Surinam X Hog Island boa that had stopped eating randomly. This had never occured in the year that I had her. After talking about symptoms to the vet, and a few very respected boa breeders, we were at a loss. The entire time I had had her she ate f/t rats only, and never had any problems of any kind.
I took her in to see me vet and had blood tests and a stool sample taken. She was found to have round-worms and a high red-blood cell count. I was SHOCKED. She was administered medication for the worms, a one time dose. The vet said this usually isn't a cause for concern in itself, but it is possible that the worms caused other problems. Blood tests came back, and she had kidney disease. She died 48 hours after the vet visit.
Now Kidney disease is often a result of being treated with antibiotics earlier in life. I did not have her for her first two years of life. She was most likely fed live rodents at one time, which caused her to contract the round-worms. And it is likely that she was later given antibiotics to remedy the problem. While she seemed fine, this was an inevitable formula for an early death. Usually young snakes treated with antibiotics that DO get kidney failure die before they are five. I will note that often there is NO choice but to use antibiotics, it is not that they are all or always bad.
As a new owner you seem to care very much about your new pet, I urge you to take this post into consideration.
One final point. Rats are usually preffered to mice because snakes who are fed rats tend to grow at a faster rate than mice. One is not necessarily more nutritious than another. Also, when your ball python is an adult it will be much easier to feed it one rat than 4 or more mice.
Good luck with your new pet!