Letterboxing is allowed BUT it requires a Special Activity Permit which costs $30 every three months to maintain. Source of information was an e-mail from Crowder's Mountain State Park (Gastonia, NC) Superintendent, Larry Hyde on 12/14/06 after pulling letterboxes from the park that did not have permits.

NC State Park Special Activity Permit

Documented policy is no longer located on any state park website but can still be viewed here.

Yes. After answering this question repeatedly, it just seemed to make sense to put one together. Every box listed in this guide as well as the tips to each box has been done only with permission of each planter. By listing it in Wiki, anyone can update it to keep the information more up to date (or add or remove their boxes as they see fit). You can also check the revision date of this question listing to see when the guide was last updated.

MAINTENANCE WAS PROVIDED TO ALL BOXES IN 3/2012
Most of these boxes were planted when the USNWC was opened. The trails have been extensively changed since then and many clues became outdated. Any clues that have not had a chance to get updated by the planters yet have additional notes after the clues. Planters are all aware of this guide and may update it as clues are updated and the additional notes are no longer needed

Clue Guide for the trails at the USNWC

This is the largest man-made white water facility in the world. You can pay money for a ton of different activities OR pay nothing more than $5 to park and have access to all the trails for free.

Important information about the trails
1. ALL of the trails are for mountain bikes. Be aware of this and get off trail when they come through.
2. BEFORE YOU LEAVE CHECK THE TRAIL STATUS HERE. When they are wet they close them to preserve the trails.
3. TRAIL MAP: $2. I have a photo of one that has the trails you need HERE.
4. Parking is $5.
5. HOURS OF OPERATION: HERE

Important information about the facility
1. DIRECTIONS: HERE
2. There is a full bar and restaurant there as well as restrooms and showers.
3. ACTIVITIES:
Whitewater rafting
Whitewater Kayaking
Flat Water Paddling
Climbing Walls
Canopy Tours
Hiking/Mountain biking trails (free)
Eco caching
Zip line
Classes/summer camp/group activities
4. EVENTS: They have regularly schedule bands that play there and other events.
5. PETS are allowed on leashes only. They are even allowed in the outdoor section of the restaurant. The restaurant even provides drinking buckets for them. :)

Important information to plan your day on the trails
I have broken the guide into THREE hiking loops. IF you want to do ALL 3 loops in one day anticipate walking approximately 8 miles and needing about 6hrs. There are moderate short up and down grades in some sections but for the most part they are fairly flat. Since each section is a loop that takes you back to the parking lot you can decide after each loop if you want to do the next without doing all the miles in one day.

NORTH MAIN BOXING LOOP (Approx. 3 miles):
Park at the front of the USNWC main entrance

Go through the USNWC main entrance. Then go to the right around the rapids to the North Trail MTB Entry.
For your first box you will be picking an end of the Figure Eight Loop to find:
Three Secrets.
After that box, go back out of the Figure Eight Loop and continue on the
North Main trail for
A Stamp in Every State: North Carolina Pug and
The Den.
After exiting out of the trails back on to the walk around the rapids, go right to the NEXT trail over for
PADDLING.
Important update to Paddling clue:
The trail you are looking for is no longer marked (the clue states the USNWC Flatwater trail). You want the UNMARKED trail BETWEEN the North Trail MTB and the Flatwater trail to the kayak launch. There is NO RED trail, but there is a trail to the left. Stick with the high road when you have a choice of trails after turning left. There is NO "bridge of stone repairs"-- it's now a wooden bridge. If you reach the canopy netting you went too far.

SOUTH MAIN BOXING LOOP (Approx. 3 miles):
Park at the far back corner of the park lot at the trail head (look for the flags marking the trailhead)

Follow the South Main trail.
As you follow the clues to
U Decide take note of the RED ARROW. You will be coming back to this point!

Now go back to that RED ARROW and take the path DOWN for
Donkey and Me.
Continue on to the Carpet Trail for
Movie Madness: Betelgeuse.

Then back on the South Main Trail and continue on for
Something Smells Fishy

LAKE LOOP BOXES (Approx. 2 miles):
Park at the far back corner of the park lot at the trail head (look for the flags marking the trailhead)

Technocow

Toilet Bowl.

DRIVEBY ON THE WAY OUT OF THE PARK (only box that is accessible without a parking fee):

SIGNS...

No.
The policy can be found here and assumes that it applies the same to letterboxes as it does to geocaches.

No.
The policy is here and assumes that it applies the same to letterboxes as it does to geocaches.

No.
The policy can be found here assuming that it applies the same to letterboxes as it does to geocaches.

Letterboxing is allowed except in Wilderness areas or Research Stations.
The policy is here.

The Mecklenburg County Park and Recreation Department (MCRP) have created a permit system for letterboxes (and geocaches) planted on their property, which includes Big Rock, Evergreen, Latta Plantation, McDowell, Reedy Creek, Ribbon Walk and West Branch Nature Preserves.

Letterboxing (and Geocaching) Policy (PDF)
Letterbox (and Geocache) Permit (PDF)

Big Rock Nature Preserve Placement Options Download PDF

Evergreen Nature Preserve Placement Options Download PDF

Latta Plantation Nature Preserve Placement Options Download PDF

McDowell Nature Preserve Placement Options Download PDF

Reedy Creek Nature Preserve Placement Options Download PDF

Ribbon Walk Nature Preserve Placement Options Download PDF

West Branch Nature Preserve Placement Options Download PDF