Mice are one of the most nutritious food that can be fed to a snake. Gut loading mice is definitely a thing, though not as simple as gut loading insects. Insects can be gut loaded within 24 hours, while, to achieve a discernible increase in the nutritional content of a mouse, relatively long term care is needed, 2 days before feeding will probably not be enough. This isn't too much of a problem though, since snakes don't require as frequent feedings as most other animals, and so a longer duration of care is somewhat manageable.
Being a very nutritious food, supplementation isn't really required for a mouse. A juvenile snake can definitely benefit from the added nutrients,but can also do normally. The question here is whether the supplementation is worth it, in this forum, the user Reflex refers to a snakebytestv episode where rodents with vitamins where given to some snakes, while others were just given rodents.
There was a snakebytestv episode once that showed the results of
feeding all different things to same-sized corn snake babies, and if I
remember correctly, the results were that the ones given vitamins on
their rodents grew slightly bigger, but not enough to make a
difference in the snake’s health. It was pretty interesting, if I can
find the episode it was I’ll post it here.
After some research, I found the video to most probably be these two: preliminary, and the followup (result).
It seems that gut loading mice is a bit of a chore, and at least in this example, it gave discernible but not tremendous results. There is also the concern that the mouse can fight back and damage the snake.
We highly recommend that you only feed pre-killed mice to your herp.
Live mice can fight back while being eaten, gnaw on your snake if left
in the cage with them, or be too hard for some younger herps to catch.
Attacks by live prey can disfigure herps, and they have been known to
scare a herp off of his natural diet of prey animals. If the mouse
fights back while your herp is eating it, it can bite through the
herp's mouth area, puncture his eyes, and cut through his tongue.
Feeding pre-killed mice can alleviate all of these concerns. (
And of course there is also the added advantage of the vitamin supplementation to some frozen mice. I personally have no experience with snakes, all info here is based on the information I've accumulated through watching and reading. If it was up to me I would stick to the frozen ones and make a live mouse an occasional feeding to mentally stimulate the snake and encourage a more natural behavior. Before feeding the mouse live gut loading it is definitely an advantage if you consider it worth the relative effort.