Below 85c there is nothing to worry about long term.
Hmmm for me it's more like below 70C and 50% fanspeed. But to each his own.
Temperature variation is the real card killer. I'd agree with anything < 80-85C, but some cards just don't stay that cool even standing alone with good ventilation and room-temperature ambient environments.
It's more important to make sure your temperatures aren't swaying significantly on a regular basis, the (small) expansion/contraction will eventually stress the metals in your GPU, leading to reduced life.
A stable 80C is far better than swinging between 60C and 80C every day.