I add a small amount of Vit E to most of my batches so that kind of covers if I need one and folks like that it's in the soap. Sort of a 2 for 1, bang for your buck.
I should say I also use 50% OO and I also use Canola oil so I do worry about DOS and that's why I got Vit E to begin with although Vit E as a preservative gets mixed reviews.
I have a milk soap that is over 5 years old that is still in perfect condition, no nasties or DOS, I am very surprised it hasn't gone bad yet, it was made with the 3 basic oils and GM (my very first soap)