I think your "pro" photographer here was talking referring to times when studio flash is being used. In that case you may be able to get away with it most of the time. For outdoor/nature/landscape use, sometimes but not all the time. ISO is a tool, a powerful one. If you shoot in manual you know what I mean. A good workflow would be start with ISO, for quality purposes, choose the lowest you can get away with in the light conditions, then set your SS and aperture, then come back to ISO if you can't get those two where you want them. Of course, none of this is set in stone. For example, when I shoot with my 85 1.4, I almost always shoot it wide open, in that case, if I am outside on a bright day, I'll set at ISO 50-100 because I know I'll be at the top end of my SS.