I personally only soak if one has a bad shed (I only have one girl that consistently needs help with her shed - she's just not a good shedder).
I don't soak for any other reason. The one time I had mites on a pet store rescue, I did not soak, I treated the enclosure with Provent-A-Mite and then preventatively treated the rest of the snakes' enclosures.
However, I do believe that some people will give their snakes an overnight soak to hydrade them before shipping, if they are selling them.