A good rule of thumb I've heard is Valentine's day for the most of the south. I just usually prune mine to my own taste and always am rewarded with lots of blooms. I know antique roses don't like to be pruned severely but hybrid teas do. I don't think you can fail with a knockout...cut it to your own liking because it will grow like gangbusters anyway. I wouldn't prune any once blooming roses for fear of cutting off potential blooming stems; wait till after they bloom. Just my own experience, I'm no expert by far.
Jen

As a rule of thumb, roses are dormant when they have lost all their leaves. This rarely ever happens in the deep south, so we prune sometime during the winter, most commonly in February. I have pruned as early as mid December and as late as late February and it did no harm that I could ever see. Around here, the wisdom is to prune around Valentine's Day.

It helps to understand how roses grow. If this is too elementary for you, please forgive me. I'm just trying to be thorough and clear.

First, the branches, properly called canes, have joints (nodes) all along them. If you look carefully, you will see slightly swollen to quite prominent bumps (buds) emerging from those nodes. (They tend to be red colored on many of my bushes.) Notice that the bumps occur on all different sides of the branches. These buds are potential branches. They will grow straight out of that bump and keep growing in that same direction.

You want to keep the center of your bush somewhat open and free of congested canes. This ensures good air circulation that minimizes disease, and allows sunlight to reach all parts of the plant for good flower production.

So, you want to always prune a branch just above (about 1/4 inch) a bud that is facing away from the center of the plant. If there are buds below it that face into the center, you can rub them off with your finger.

When you cut, it's a good idea to make an angled cut, so that rainwater will run away from the bud you are encouraging, rather than right onto it. The bottom of the angle should be about 1/4 inch above the bud. If your plant has disease (or even if you are suspicious), it's a good idea to wipe your pruning shears with alcohol or a disinfecting wipe after each and every cut. That way, you won't spread disease to healthy canes.

When you cut a healthy rose cane, the cross section of the cut will have 3 layers: the large center is white and clean looking, then it's surrounded by a thin green layer, and then the outer skin ("bark" so to speak). If that center wood is black or brown, the wood is bad. Cut that branch back to healthy wood or remove it entirely.

Big, fat healthy canes will produce more blooms and more canes of their kind. Skinny, wimpy canes will produce few, if any, blooms, and more of their kind too. Keep the fatties, remove the skinnies. (Roses, not people. :)

So, with these things in mind, here's how to prune shrub roses:

1. Remove all dead branches. If the wood is grey and brittle, it's dead. Cut it off at its beginning point. If the beginning point is dead too, cut that branch back to its source, or to alive wood above an outward facing bud.

2. Remove all skinny, spindly wimpy branches. If they grow off big canes, remove the skinny, keep the fattie.

3. If you see branches that are crossing and rubbing against each other, remove the skinnier one, and keep the fatter one.

4. Clear out the center of the plant so there is plenty of room between the canes you are keeping.(I like to keep a ring of fat canes that are spaced evenly about 4 to 6 inches apart around the center of the plant.)

5. Now, shorten the big healthy canes that are left, again, cutting above an outward facing bud.
Typically, you would remove from 1/3 to 1/2 the height of the canes. When I'm finished, I like all the remaining canes to be fairly close to the same height.

I have found that alot of antique roses that have been lazy bloomers respond well to really hard pruning. In that case, you can count up from the base of the cane 3 to 5 buds and cut above that point.

(Use this hard cutting back with caution on hybrid teas. You don't want to ever cut below the graft, or you'll lose the beautiful flowers you paid for. Leave these canes longer.)

cottage girl is right. This is not a life or death thing. Roses are really just weeds that have really pretty flowers. If you cut too hard, or perhaps not perfectly, they will recover quickly and you'll have another chance in short order. You can even do some corrective pruning, following these guidelines, during the season. Just use a lighter hand.

One important thing to keep in mind is when is the last hard frost in your area. Remember that pruning initiates new growth, you do not want to prune too soon an cause new growth that will be killed by a late hard frost/freeze. If that happens, what you will get is what's called "blind shoots" that will grow but will not produce flowers.
In the Charlotte, NC area the serious rosarians wait until mid march to prune. That may be a littel early for your area, but I would suggest you do a google search for your local chapter of the american rose society and ask a local for advice.