I've heard the same advice. I'd wait until this time next year and then do it as a rule from then on. They do get woody and sparse if you don't prune them hard each season.

Don't cut them to the ground, just cut them back to 18'-24' in height. Some varities don't take kindly to severe pruning but they can all take being cut back. If they've only been in place for a year I wouldn't cut them back at all. I give my shrubs at least 2 years before I start pruning although I do remove any dead, damaged or diseased branches as needed. I'd give them another season to get established.

I cut mine done to the ground when they get out of hand, and they bounce right back with no problem.

We leave in southeast NE...what is the rule of thumb for pruning butterfly bushes?