I think it depends (BTW: hello again!). For instance, there's a trail that's maintained by my high school (on a rather large, boarding school campus) that has mud holes, and ways to go around the mud holes. You always see bike tracks, horse hooves, etc going through the mud, as well as around. They keep adding dirt to the mud holes, and it never seems to deteriorate much, but as it's in the woods, they never really dry out either. This to me is OK, and I've gotten super-sloppy when hitting all the mud on the trail (i.e. you can't see the color of the bike), but I do find that trails that aren't maintained like that, you shouldn't ride them if you're obviously damaging it for others. I.E. if there's significant mud, and your tracks are 1/2" deep or more? Don't do it.

YMMV.