Any DOWNSIDE from putting it on for the whole cook?
My strong impression is that it is almost always applied right before searing (as thin a layer as possible). Essentially the emulsion enables the oil to adhere more efficiently to the moist meat, and the added protein etc contributes to browning. In the sense that many people feel oil in the sous vide bag can extract flavor from the meat, it might actually be somewhat counter-productive to apply before sous vide cooking.
I’ve been using this method for several years now. I have not been disappointed with the results thus far.