In many of the maps I create for history student use I ask that comparisons be made across time. For example, compare population density in a particular county across several decades. In order to make an effective comparison a common set of classification values is appropriate. Students may start off classifying the first decade by Natural Breaks, Equal Intervals, or Quartiles. For the other decades, though, a Custom scheme would be great so that values from the first decade could be used throughout and comparison facilitated. If such an addition to ArcGIS Online is planned, great. If not I would like to strongly suggest it.