My heat sinks run at 120F max, and have been for almost 5 years. My LEDs are still working fine, but the heat will make the LEDs degrade faster over time.
When I moved my lights to a different tank I accidentally shorted out some of my cool white LEDs (1/3rd of my total LEDs). I bought exact replacements and noticed the brand new LEDs were noticeably brighter and "cooler" color spectrum than the LEDs I was replacing.
Just a matter of if you want the LEDs to last 6 years or 10 years. My LEDs are 3W CREE for what its worth, and I only run them about 5 hours a day full strength with a 2 hours fading up and 2 hours fading down.
Personally I think its worth it to try to keep the temperatures under 100F. I regret cheaping out on my heat sinks, I intend on replacing my whole LED setup in about 2 years when the LEDs are nearing the end of their life.