Jan, testing the software for reliability of conversion is difficult. Gretag Macbeth publishes a disclaimer where they state that they do not support the freeware, so they can't be responsible for the accuracy of the conversion numbers. I have the most recent download (version 6.5.1) and the "about" dialog provides the name of the engineer at GMB who wrote the app -- so at least we have someone to go to if we run into a real problem.
A real-world test would involve using the app to convert a set of Munsell code to CMYK, printing the CMYK output on a profiled printer, then comparing the output to Munsell reference chips under controlled lighting conditions. I don't have the reference chips and acquiring them would be expensive.
I'm going to proceed under the assumption that the Gretag Macbeth freeware makes accurate conversions. Since the program author is also a GMB employee, we have to assume he is actively working in this area and knows what he's doing. After we've produced some results, perhaps we can contact him, explain how we're using his program and get his input on the accuracy of our results.
One final thought, we also have to assume that whoever created the Munsell codes for the Methuen plates did an accurate job. I think there's a greater chance that the Methuen-Munsell conversion codes may be slightly off in some cases than that the Munsell freeware is inaccurate.
The joy of working with color