It all depends on the rose. If a grafted rose is hardy to zone 6, it will have no trouble staying alive in zone 6. If Dr. Huey takes over, it's either because the suckers from the rootstock were not removed from the rose or that the scion was never meant to live in zone 6. Dr. Huey, on the other hand, is the most common rootstock and it is hardy to zone 6, so it will continue to thrive even if cold weather kills the scion.
The huge bushes up the road from you may be growing on their own roots now, but if they were purchased from a nursery in the early seventies, they almost certainly were grafted when they were planted. If the bud union of a grafted rose is below the ground, there's a good chance that the rose will start growing on its own roots, especially if the scion is more hardy than the rootstock. In the warmer zones, we keep the bud union an inch or two above the ground. That way, if we ever do get suckers from the rootstock, they're easy to identify and remove.
I prefer hybrid teas and other modern roses, so I've stopped buying own-root plants. The roses I like won't grow well on their own roots. Granted, my garden is in a warm zone, but I have an 84-year-old grafted hybrid tea in my garden. When I bought a house in 1974, one of the roses in the garden was President Herbert Hoover, a hybrid tea. The previous owner had planted it in 1932. It's the only rose I dug up and took with me when I moved a couple of times over the years, and it's still in my garden today -- a healthy rose that blooms profusely and stands about 6-7 feet tall.
Several years ago, I started worrying that it would die someday and I acquired a replacement, an own-root President Herbert Hoover from Vintage Gardens. That rose required lots of TLC, never grew beyond 2-3 feet, bloomed only sporadically, and gave up the ghost after 4 years. It was a waste of time and money.
