How Accurate and Precise is libm.a?

Chad asks: "I am looking into doing so molecular modeling on Linux platforms because they are cost effective. After running some tests, I find errors, albeit small, in the results as compared to similar tests on SGIs or SUNs. I have heard about this in the past when talking to some professors but I never thought much about it until now. Knowing that errors propagate and grow (especially after weeks of computation), I want to know what I can do to avoid problems with the math library. Is it a problem? Can it be fixed? Am I over reacting?" Can anyone offer up some information on this? Has anyone actually stressed tested libm.a?