Whenever we drive by, we try to stop and visit. If you have American Rose Society membership and have your membership card with you, entry is free with the reciprocal gardens set up. (Also good for Morris Arboreteum, just an hour east)
Generally, for a first visit, I'd aim for first bloom. But that depends on winter not being a killer (and we don't know yet what winter will be.)
Must sees are the classic arches at the base of the garden bedecked with either Dr. W. van Fleet or New Dawn. The roses around the pond looked much better last year than in previous years, some rosarian/gardener has been working with all of those Red Fountains.
They have mass plantings of modern roses and a few old roses (outside the pay area) in shade in what's called "Mrs. Hershey's Garden." They do have a butterfly house in the rose garden and they destroyed the Old Rose area and built a child's something or other on top of it; as of last June, the Old Roses hadn't been replanted anywhere.
Their gardens are evolving. They used to have masses of minis; I didn't see them in June.
In the fall of 2004 and 2005, they had problems with rust on most of their roses. I don't know if the rust is under control or not.
Our visit in 2007 was the very last week of June and seemed to hit first bloom fairly well (although they were still replanting beds that had been destroyed by winter.)

I would say sometime in the first three weeks of June would be peak. Maybe earlier if we get hit with a heat wave. As an FYI, the Reading/Berks Rose Society holds their rose show not far from the gardens on the second Sunday in June.