Just from experience - here - I have ruined so many LF negatives by being concerned about over stopping. I wind up with DOF problems when I am reluctant to stop down. Since I started stopping down without fear - the negative quality has gone up. I have yet to see it in my own work where lens diffraction was in any way apperant. Of course in cute little film it might really be a consideration but for MF up to 8x10 - Stop it down - Get the dof - I don't have scientific charts or data for all this - Just a drawer full of expensive experience. I believe it was the"f64 club" not the .... f - can you get that down to f22 'cause I might get difraction club.
Ok - it drove me insane and now I am recovering from that. sorry for the rant -