Because the data in the forecasts predicted, most often as a number, the round will probably result in change. So why in the spreadsheet is a good choice, but it might be a little problem in the round when displaying results.
Because of the projected forecast figures which most often are the rounding of the analysis may result in a change. So the calculation in the spreadsheet is a good choice. But it may be difficult to round off a little when showing results.
Because of the situation forecast usually numerical analysis which rounding may cause changes, so calculations. Spreadsheet is considered to be a good choice.