I have never worn a steel-boned corset for an extended period of time (more than six hours). I tried corset training for about three days, a few years ago, but it always seemed like life got in the way.
I have worn a steel-boned corset for costumes, and I find that it actually makes me pull in my core muscles more tightly. Doing that is what makes wearing it more comfortable, relatively speaking. I've also found that, for about day after wearing a corset for 4+ hours, my waist looks smaller, my clothing is looser, and my posture is better. I think that this happens because while wearing the corset, I have been making an additional effort to sit and stand up straighter. This effect has lasted anywhere from 1-3 days, but obviously, this effect goes away as I get all lazy and slouchy again I don't know if this effect would continue if I did long-term training, or if atrophy would eventually set in. I imagine that if you have an active exercise or dance regime, that would offset atrophy.
Dita von Teese, the famous burlesque performer, did train her waist, and as far as I know, never had any health issues from it, as she performs regularly onstage.
I cannot speak for what is best for someone else, or would work for them, but if this is something you really want to do, then obviously go slowly and carefully, and do your research