For numeric values, Cell.StringValueWithoutFormat just gives the string of the numeric value kept in data model, such as by Double.toString().
We think you may want to get the string of the double value with maximum digits like what is shown in ms excel. If so, please format the numeric value with your custom format pattern. For ms excel, commonly the maximum precision for numeric values is 15 digits.
Because Cell.StringValueWithoutFormat leads to ambiguity for users, we have made it obsolete in recent versions and will remove it later in this year. Please format double values with your own format pattern according to your requirement instead. For cells, maybe you need to check whether the value in data model is numeric firstly, Cell.IsNumericValue should be helpful for such situation.