The roses never truly go dormant.  Those of us growing or who grew roses in areas where there is no true winter say they are forcing the rose to go dormant.  What we mean is that we recognize that for roses to be healthier, we give them a period of rest by not deadheading the plant for a couple months and allowing the rose to create hips.  Note:  not all roses create hips.  The idea is to allow the plant to complete the whole growing cycle rather than interupting the cycle with deadheading. 

Many of the old garden roses from Europe need a period of dormancy to thrive, but this is only a rule of thumb.  As for other roses, it will depend on their genes.  I am not enough of an expert to give you a more complete answer.  A lot of gardening is trial and error.  Some roses will surprise gardeners by performing better than expected.

Smiles,

Lyn

In Wasco, California, where the majority of roses for the American market are grown, they often have to withhold water and spray a heavy oil to burn off the foliage to force the roses to stop blooming so they can be dug, stored and shipped. As a general rule, Old European Garden Roses tended to go "dormant", or as much as they would go dormant, with cold. They evolved and were selected in a harsh environment where the ability to have their soft new growth checked by cold weather assured their survival. Ever blooming roses, such as the Teas and Chinas evolved in an environment which was most often less harsh, as far as winter temperatures go. They are pushed into dormancy by lack of water, and brought back into growth by the availability of it. So, the answer to your question would depend largly on what type of rose you're considering. My best guess would be if the rose is once blooming, it would tend to need more of a dormancy period, meaning one in which it isn't forced to grow and attempt to flower. Such roses would be the once blooming ramblers, many of the Old Garden Roses from Europe (Damasks, Albas, Centifolias, Bourbons, Hybrid Perpetuals) as they tend to have more of the cold climate rose genes in them. Of course, as the classes developed, more and more of the everblooming China genes were incorporated, but you can often tell what is needed by what the rose looks like and how it perform.  The Portland roses tended to be combinations of the European OGRs and China roses, with more of the China habits dominating. They look like the Europeans, but perform more like the Chinese roses, and more like our more modern roses (Hybrid Teas, Floribundas, miniatures). If the rose tends to want to continue growing and blooming, I'd say the "winter rest" isn't that much of an issue for it. Of course, when you prune, you do force it into a semi dormant stage, as it's not creating the level of nutrients due to the removal of the foliage, and stored nutrients present in the removed wood.

When you read "withhold water in winter months" in the cultural description of a plant, it's with the intention of forcing dormancy for a rest. With your modern roses and those which perform more like them, allowing them to slow down in growth by not watering as heavily as you had in the warmer months MAY help them live longer. Or, it may not. There are too many variables to know for sure without more experiementation and study concerning the particular roses you're considering as well as your climate, culture, etc. Pretty much, you have to let the rose show you where it wants to go.