Start to worry once things get on the exiting side of 80C, ideally temps would be less than 70C, as above that the card downclocks itself slightly. The difference is only a couple of Mhz though, won't affect FPS what so ever.
You can safely go over 90c, but anything over 80c is telling me that your case airflow isn't good and/or that your ambient room temp is high. It's hard to compare your temps to professional reviewer's temps because those are usually done on an open test bench which is cooler than your closed PC case. So, don't be worried if you see a review with temps at 60c or less and you are in the 70c's.