I've pre-soaking for a couple years, everything from 35mm rolls to 4x5 sheets, 30 seconds to 3 or 4 minutes. Noticed a change for the better using a pre-soak. Haven't been able to notice any difference the time in the pre-soak makes.
Started out using DI water for everything, now I use tap water escept for developers I make up in bulk. Again, no difference noted between tap and DI water. But, tap water varies greatly from well to well, so there's no guarentees associated with it.
That may be one reason why manufacturers now recommend against it. The corporate legal-beagles hate being held responsible because someone's precious negatives failed because the consumer wasn't warned about the varying results due to local variations in water.
Pre-soaking has been used longer than most of us have been alive. Emulsions have changed over the years but I don't think there's good evidence of pre-soak hurting anything, given no major contaminents in the water.