I have my shuttle cube celeron 2.0 running for almost 2 years now 24/7 powering my webcams and weather station software. Runs great. before that, had a dual celeron system. The celerons' were only 366's but overclocked to 550. The heat effect did incur a toll as towards the end of life (4 years) the chips could only handle a max of 500mhz overclocked. Although they had an adequate heatsink/fan, the room temps would soar above 90 degrees every day of the year. However, I could care less since after 4 years this system was just a slow piece of junk anyhow (hence I replaced it with a celeron 2.0).
I therefore wouldn't worry at all about heat effects. Yea, provide adequate cooling but don't need to go overboard. You're not going to keep a 10 year old system operational 24/7 anyhow.