A nice thing (at least in Poser 7, and I'm fairly sure it works the same in previous versions) is that if you change your setting in preferences, it will automatically convert the numbers in the Material setup - so if you prefer to work in Meters, you can simply set your default measurement to Inches temporarily and make whatever number setting required, and then change it back (you don't even have to leave Materials Room to do it).  I am pretty sure that saved files store the number value in a basic unit type that is independent of your measurement unit, meaning all your old content should render the same if you ever do change your preferred measurement unit.

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