Jobo does not recommend a prewet with Xtol because Kodak published rotary processing times WITHOUT a prewet.(posted 8732 days ago)Jobo found that in general a 5 minute prewet compensated for the difference between intermittant agitation in a small tank and continuous agitation in a rotary processor. It allowed them to make a general starting recommendation for any film/developer combination.
All of this is documented in back issues of the Jobo Quarterly, which you can buy, or read on line at the Jobo US site.