Whether or not, or how much a plant needs to be flushed depends solely on how the plant was fed to begin with. If you have salt built up from overfeeding with synthetic or organic nutes, then you need to flush for sure. A properly fed plant IMO really doesn't need to be flushed because it should use up whatever food it has before it's done, but not too early that it starves at the end.
Clean burning grey ash comes from properly fed AND properly dried and cured bud, both have to be done properly. In my experience, organically fed plants need less "flushing" than synthetically grown bud, and it NEVER tastes as good no matter how well it'sflushed and cured. Not that you can't get very good tasting herb from properly grown synthetics, because you can, it's just a little harder to do.
If you have any doubt, or question as to whether or not to flush, do it, it can't hurt as long as your plants aren't already turning yellow like they're running out of food.