Pfff, I only mine LTC and Im happy if my cards are under 100c. But they are kind of cramped up and OC'd to bejeezus.
My understanding of GPU tech is that the chips can run MUCH hotter than a CPU for long periods.
For a CPU, 90c would be too hot but for a GPU 90c is totally fine, but I have to set fans to 85%+ to get this, so fan life is reduced.
Running gpus at and over 90C will kill them quickly, at least in my experience. I was running them at ~85C and they were ok until I pushed them over 90C.
Now, I'd recommend staying under the low 80s.