There are a few points that need to be addressed here.

1. Some users do not want nulls to always be forced to the bottom of the sort. (see sortService.sortData assumes that users want to sort nulls to bottom  #545)
2. When reversing the sort, nulls can creep back up to the top of the sort. (see https://github.com/angular-ui/ng-grid/blob/master/build/ng-grid.js#L447-L451)
3. When users provide their own custom sortFn, they do not get the option of handling nulls.

I'm currently working with a date sort on a large dataset that has several nulls, and when I reverse the sort of my values, I get ~100 empty cells before the first real data and I can't coerce the grid or my data otherwise to fix this problem.