Its still hanging weight your not going to change that no matter when you feed them in the 12hr window you're talking about.  High temps can cause some unusual shrink if dry lotting but this time of year its cool enough that you're not going to see any benefit keeping them on feed or water overnight depending on distance and truck weight restrictions.  If its just down the rd your truck and/or freight is cheap it doesn't really matter....feed them up.  Longer haul and freight takes a good bite out of the whole thing you can easily shrink them enough to get another animal on to pay the trucking bill. Thats how I look at it.  Just to spell everything out so everybody understands.  In the meat is strictly by hanging weight, on the grid is by grade and then depending on grade there is a value assigned to the grade and live weight is on the ground with a percentage of shrink.