I originally requested this as an enhancement request in 2002 (CQ00152782), thought it was time to submit it again!
ArcGIS must be the only mapping software in the world that doesn't provide an option for suffixes or prefixes on what ESRI call measured grids. For example mE for eastings and mN for northings, so instead of your map reading 253000 it says something useful like 253000mN.
Why would this be so hard to implement?