Ive found that washing my hair/face and drying it/fluffing it up makes the gunging feel heavier and slimier - in my recent session I puffed my soft clean hair up before clobbering it with sweet sloppy pies
I washed during a session, cleaned up and added more slop in which to wallow around and pour over my head. Never done that before. Added a whole new twist.
It varies for me - the couple of times I've been to gunge events with a group of people, starting out clean and fresh feels like basic courtesy, as I'd do in any setting where I'm physically close to others. If I'm heading out for solo mud I won't always shower that morning as it seems a little superfluous but I do like to have my hair and beard recently trimmed, it just helps me feel confident sharing pics and video, and also feels good on the shaved sides of my head.