OK, so I'll start from the low end of the high and work my way up...
Everything I post will be uncropped (unless otherwise noted) so that the effect of noise will not appear exaggerated. All EXIF data intact.
These three all at ISO 800 with the Canon 5D. The first two with flash.
On #1, since I almost always expose for the background (ambient) when using strobes, I needed a high ISO to maintain the shutter speed necessary to handhold a 180mm macro while maintaining the f/11 for the DOF I was looking for. The flash was only for fill.
On #2, although the strobe is the main light source, I needed the high ISO to bring up the lights of the strand.