While it is common practice, it is a horrible practice and I do not agree with it, especially for garters. You can quarantine without neurotoxic poison. Baby snakes are very susceptible, garter snakes are especially vulnerable because they are constantly in and out of their water which allows the poison to soak into their skin.
There have been numerous documentations on this forum of disastrous results from prophylactically treating new baby garters. Results are usually twitching and spinning for a day or two, followed by death. Very cruel if you ask me. Like I said, should only be used as last result when mites have been confirmed. IMHO