I've tried using the GetFractionBase() method, but that just returns -1 if the dimension uses document defaults.  Sadly, the majority of the dimensions on our drawings default as fractions that don't round to nearest.  The only time we need to override the defaults is when the dimension rounds to a fraction, but we need it to be decimal.  Until I started writing this program, we never cared what was going on under the hood, just that the dimensions were correct on the drawing.