Pickering gallicas and rugosas will do really well here in zone 6b with graft burried approx 1 inch below soil level or even at soil level. They are quite hardy and You don't want it deeper then this for gallicas and rugosas because they will become own root and will produce suckers. HPs as well as bourbons, modern roses you can plant with graft 2-3 inches below soil level. These roses are usually not suckering even if grown on their own roots.
Olga

Thanks so much, Olga, and also thanks for the info on my gallicas blooming season question. I'm adding this info to my rose info file.

The roses described are going to be hardy in 6a except for maybe certain HPs. The rootstock may function better if not buried unnecessarily, say the graft at grade level for the suckering roses and slightly covered for the HPs. With HTs and some modern shrubs, all exposed wood will be damaged at -5 or -10 F. Deep planting protects the graft without the trouble of mounding for winter, which can also encourage cankering of the canes.

A few hybrid rugosas such as 'Topaz Jewel' are more tender.