Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $236.83   6.486%   $252.1908   $252.19   $252.20   $252.20  
  Purchase #2 $104.76   10.751%   $116.0227   $116.02   $116.00   $116.00  
  Purchase #3 $26.63   3.220%   $27.4875   $27.49   $27.50   $27.50  
  Purchase #4 $115.58   5.136%   $121.5162   $121.52   $121.50   $121.50  
  Purchase #5 $121.56   6.673%   $129.6717   $129.67   $129.65   $129.70  
  Purchase #6 $41.58   12.511%   $46.7821   $46.78   $46.80   $46.80  
  Purchase #7 $204.38   9.706%   $224.2171   $224.22   $224.20   $224.20  
  7 purchase totals = $851.32       $917.8881   $917.89   $917.85   $917.90