For a muddy course you can run as low as 30-35 psi on a clincher, the trick is to adjust it for the terrain.
No point using way low pressure if the mud is so bad you're going to run it anyway.
Yesterday's race was a mix of pavement and dry-ish dirt/grass and total sloppy mud and standing water, so I set my pressure for the dry stuff and ran it when the going got too slippery.

I see a lot of guys using the paved and flat sections for recovery, this is where you want to be going fast, big ring it and hammer, you can recover when you're done...