Cindi ...
You already know that modern roses do not go truly dormant. They have slowed down, but true dormancy is when a plant moves all of the moisture and nutrients out of the cells of the top growth and sends them down to the root zone for plant survival.
Die back ... gross generalization ... is caused by the cells holding too much moisture and freezing. Since modern roses are not generically programmed to move that moisture out of the top growth, planting them at 50 degrees is not going to harm them.
Photosynthesis in roses slows down at 70F, so they are not going to be pushing growth. In Zuzu's climate, this is not as detrimental as it is in yours because the plants will have warmer soil for a longer period of time both now and in spring and will be able to grow the feeder roots it needs. I don't know how that works in your climate.
As usual, I am back to talking about growing roots. It does make a difference in how a plant takes off and how healthy it is in the long run. However, I have never grown plants using multiflora rootstock. Maybe someone who has grown roses in your colder zone can give you better information about that part ...