I'll give you my $.02 worth. Our last trip to Hazel was several years ago before the marina when up on their prices. It was mid-May. We went over on a Monday and came back on Friday. We stayed at Bone Valley where there was one other group of three. On the way up Proctor and Sawdust Pile were empty and Sugar Fork had one tent on it. There was also a group of 5 or so at Calhoun.
On Thursday evening a group on horseback came into Bone Valley, but camped away from us over a small hill and out of site. On our way out Friday, Sugar Fork, Sawdust Pile and Proctor were all virtually full. Ran into a friend from Charlotte who said he was on a trip with Jesse Brown Outfitters, so that explained some of the crowds. On the boat ride back to the marina, the driver said they alone had over 50 people over on Hazel.
With all that said, go anyway. Just don't expect to find a lot of solitude in the first 5 miles of stream over the weekend. If your knees will handle it, walk above Bone Valley to fish - and don't pass up Bone itself as "too small". The trail up to Bone Valley rises about 500 feet from the tailhead - a very easy hike. Even with that many fly fishermen on the creek, most of them fished within a mile or so of their campsites. Just like any stream in the Park, the further you get from the trailhead, the fewer people you'll see.