Answer: Pruning to the ground generally means as short as you can, in pratical terms this will be anywhere from an inch to six inches depending on what plant you are dealing with. Cutting back "to" a given height means leaving that much. Pruning instructions can vary substantially depending on the desired end result, the region, the individual plant, and the season, so it is no wonder you've read all kinds of conflicting advice.
Blue Mist or Caryopteris often suffers fairly extensive winter damage in colder areas and so dies back naturally to somewhere close to the ground. In a mild winter this may not be the case. In my experience it is better to trim away any winter kill, then prune a bit for shape. I would not trim away any more than a third to half of the live wood, in any case. You will find that a light trimming will stimulate branching and thus more blooms.
Buddleias or butterfly bushes may also die back to the ground in a severe winter although they remain root hardy. You may opt to prune them to the ground (in practical terms this means to cut them off very short or to about six inches from the ground) for size control because they can grow very large over time if they don't freeze back. If you cut it back very hard like this, tip prune a few times as it grows in order to encourage branching and consequently more blooms. It will regrow fast from the established root system.
You may also opt to prune this shrub back as much or as little as you like. If you do no pruning it may become ungainly and straggly, so a light trim can help keep it in a slightly more regular outline. This is a matter of personal taste. I usually trim mine back to remove winter damage and then try for about an overall sixe of three or four feet, trying to prune in such as way as to encourage a good branching pattern. By the end of the season the shrub will be far over my head.
Prune both of these plants in early to mid spring.
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