Well there are quite a few independant tests that would verify that the type of thermal compound you use can have a bearing of up to 5 degrees on your chip's operating temperature (especially on hot Athlons) but I'm not going to labour the point. It's something that costs about £4 a tube at the very top end, so it's hardly a major outlay!
I had an associate of mine lose three 2Ghz XP chips on the trot to heat damage, eventually determined by the supplier to be insufficient contact twixt chip and heatsink.