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 $63.45   7.471%   $68.1903   $68.19   $68.20   $68.20  
  Purchase #2 $76.44   10.538%   $84.4952   $84.50   $84.50   $84.50  
  Purchase #3 $141.36   13.581%   $160.5581   $160.56   $160.55   $160.60  
  Purchase #4 $250.36   12.304%   $281.1643   $281.16   $281.15   $281.20  
  Purchase #5 $119.46   11.439%   $133.125   $133.13   $133.15   $133.10  
  Purchase #6 $143.54   11.232%   $159.6624   $159.66   $159.65   $159.70  
  Purchase #7 $84.39   8.347%   $91.434   $91.43   $91.45   $91.40  
  7 purchase totals = $879.00       $978.6293   $978.63   $978.65   $978.70