When I shoot that lens, I don't limit the iso. Its a personal choice though. If I get a crappy picture, oh well. It would have been worse if iso was limited since I am in manual and already at minimum settings. Sometimes I'll shoot animals in the back yard. In that case I often use flash and I do lock in a low iso.
I was shooting something else this winter and spotted this heron. I just zoomed in and took a quick shot even though I knew it was too dark. I forgot to readjust my shutter speed, way too slow for 600mm. While its not a great shot, its ok considering it should be more blurry and it came off at iso 25,600 with the d7100. Shot raw, it cleaned up pretty good.