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 $226.99   11.857%   $253.9042   $253.90   $253.90   $253.90  
  Purchase #2 $110.75   5.632%   $116.9874   $116.99   $117.00   $117.00  
  Purchase #3 $41.84   6.318%   $44.4835   $44.48   $44.50   $44.50  
  Purchase #4 $69.92   10.767%   $77.4483   $77.45   $77.45   $77.40  
  Purchase #5 $65.94   7.123%   $70.6369   $70.64   $70.65   $70.60  
  Purchase #6 $237.90   10.462%   $262.7891   $262.79   $262.80   $262.80  
  Purchase #7 $108.90   11.506%   $121.43   $121.43   $121.45   $121.40  
  7 purchase totals = $862.24       $947.6794   $947.68   $947.75   $947.60