When creating a STAR runway system so that all runways are active, do I use the TRUE heading or MAGNETIC heading to do my calculations or does it matter? I guess as long as I keep the variance between runways at below 7.9 it shouldn't matter?

TRUE - magnetic adds more math steps and more chances to make a mistake.Besides that - I know of no program which allows you to enter runway headings as magnetic.True values are used in the MS default files.