Rand,
Since I almost always print on "photo" type papers (papers that use the Photo Black inks), and since the majority of my images are not that saturated in color, I rarely soft-proof. Mainly because if I turned the preview button on and off, I would see no change. The newer printers, using photo papers can give excellent color gamut.
But, if there's a very bright saturated color, that may cause problems, usually resulting in that color getting a little darker. So, with very bright saturated colors, I will do a soft proof, and if I don't like what I see, try to make some corrections.