I wrap my pork butts on the final stage of the cook. When they are done I strain the fat and mix in a little bit on my red sauce (4-5 table spoons). I eye ball it but I try and end up with one cup of sauce thinned out. I pour that over the butt when I wrap it to rest (about 2 hours).
When I pull my pork I pour that sauce over the pulled parts so they don't dry out. I mainly do it to add mositure not for a flavor kick.