I think that's actually perfect, Lyn. We learned the exact same thing here.
One of the only exceptions I've found is with raised hugelkultur beds. In those beds the bottom of the root ball can sit at or about log level and be just fine. The same principal applies, and the logs will break down so slowly that any settling that occurs will be at a rate for which the plants can easily compensate. These are just my thoughts on the matter; my hugel beds are still fairly new, but all are doing enormously well.
I'm actually having rather astounding success now with plants I've tried many times to grow in the past, but which ultimately failed.
