Treat them like a perennial and cut the stems right back to the ground. Anytime. The stems die back to the ground each year and they bloom on the new stems that will appear.

I cut mine down to about 8-12" high, at the first sign of new growth in the Spring. This is usually around April. Buddleia is borderline hardy in zone 5, and from what I've read there seems to be some general opinion that waiting to cut them down until the Spring will help them survive the winter. Also, mulching them well after the ground freezes can help too.

I don't cut most things down till the Spring anyway. The exceptions are Peonies, some Phlox, and anything else that suffered from mildew or disease during the season.

I agree with Terrene; best to leave them standing until growth begins in spring, and the same goes for most summer-blooming plants. A lot of plants that we grow as perennials are actually sub-shrubs, and they benefit from being left alone until spring. Things like gaura, caryopteris, perovskia, mums, and most herbs, even the little ones like the thymes, all have a much higher survival rate if they're not cut back in fall.

I actually cut back all my butterfly bushes this weekend, not because it's good for them but because the ones I have are wickedly prolific self-seeders, and I'm trying to kill them the easy way. It's too bad, too, because they had pretty, silvery, felty leaflets on the branches, and they DID look nice. If I could just keep them dead-headed in summer ... but they get way too tall for that to work here.

I'm afraid that the recent snow storm "pruned" ours (read: snapped/broke most branches). In the past, we've always waited until the middle to end of March.

Am I too late to prune? I have a very tall leggy bush and I'm always afraid to "chop" anything! Do I really cut it back to the ground?

Where I live, butterfly bushes die back to the ground. I cut them back a little in the fall, but I leave something to show me the plant is there. That way, I don't accidently cut it off thinking it is a weed. The only reason to cut it back to the ground is that all the stuff sticking up is dead.

If you have nice leaves and branches that did not die back, you do not need to cut it back to the ground. You can cut off the dead branches if you have any.

Once it is growing and green, you can trim your butterfly bush into the shape you want it to be throughout the season.

Smarty, your Buddleia should be actively growing now, with lots of green shoots. I would go ahead and trim it back but not to the ground, because you would remove most of the new growth. I would cut back some of the leggy branches to the point where you see large healthy green shoots growing out.