I usually cut a little less than these rules suggest unless there is some other issue. If it looks like the canes harbour disease, I may cut more. If the rose is not growing vigorously and I think it needs more foliage before it can support flowers I will generally cut much less, sometimes not at all.
One will sometimes prune a rose severely if it has suffered from disease. A Baronne Prevost rose I recently cut from 4 feet to 12 inches has been slow to recover this year, but it has been disease-free and is much more densely foliated than ever before. Finally, though it has fewer roses, the ones it does produce are remarkable - much more dense and beautiful than before.
Pruning can re-invigorate a plant, but if one is to prune severely, remember that it takes quite a bit of energy for a plant to recover, so feed the roses well. And don't do it too often. If the rose doesn't bounce back with the same vigor, give it some more time to recover.
Some roses resent pruning: hybrid rugosas, for example, should only be pruned to remove dead wood.
Most rose cultivars bloom from new wood. Pruning generates new shoots which leads to new blooms. But some roses bloom from old wood, making pruning a little trickier. In these cases, one might be more inclined simply to remove aged canes.
Don't be afraid of pruning: usually it will hurt you more than it does the rose. That said, do consider buying gauntlet gloves to protect your forearms, especially before going after big rose plants with the big, sharp thorns.
There are a number of pruning steps
A person contemplating pruning a rose plant does well to consider first the fact that roses from the time they evolved millions of years ago until the time they were broadly cultivated much less than 1000 years ago, have gone completely without pruning. In this light it is clear that of all the attentions of a gardener, pruning may be the least essential to the health of a rose plant. A person cultivating a rose can do any of the other items on this list badly or not at all and in the case of many can succeed in killing the rose. No matter how you approach the pruning task your choice will not be terminal to the rose.
This said, pruning well can provide many benefits and pruning badly can set a rose back. Deadheading prolongs blossoming on remontant roses. Removal of dead wood decreases the chances of disease. Thinning increases the vigor of the remaining canes. Shaping increases the aesthetic appeal of the rose plant.
The first thing to consider is when to prune. Don't prune young plants. I rarely prune a rose before it is well established. That's generally at least two years, and it may be more than five. It depends on how vigorously it is growing, and on what size of plant you wish to end up with.
With once-blooming roses it is usually best to prune right after they are finished blooming. This will allow them to build up over the summer for the bloom the next year. Mostly this will be shaping and removal of weak and diseased wood.
For remontant roses, the very best time to prune is just before the plant leafs out. Pruning stimulates growth, so this is an ideal time. Frequently one can tell this by seeing nodules form on the surface of the canes where the canes will be sprouting leaves. This is a busy time of year, so one can also choose to prune when the roses are completely dormant. In England and Carolina this might be early January. In the Northeastern US it might be mid February to March. In the hottest parts of Texas, roses are never completely dormant, so a good time to prune is before fall's cool weather sets in august or September. One can also prune in January.
Deadheading, of course, is done after a rose blooms. If the rose is not remontant, deadheading does little more than make the plant look slightly cleaner. If you like the appearance of hips, then it is important not to deadhead after the fall bloom period.
The second thing to consider is how much toprune away. Every rosarian has her own ideas on how pruning should be carried out. Some rosarians advocate cutting a plant most of the way to the ground. Others advocate minor trimming. And each cultivar responds differently to pruning. Some thrive on it; others languish after harsh pruning. Several rules of thumb exist: