Is the a local mountain biking club? Most mountain bike clubs have that sort of information on their web sites. Some use a red light/green light system.
you can look up your local imba affiliate by state here
http://www.imba.com

It really depends on what kind of soil you have and how fast it drains. Riding in mud is really destructive. People either go around holes and widen the trails, or it just tears up the soil. Your local club can guide you.

irulan