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 $19.85   6.488%   $21.1379   $21.14   $21.15   $21.10  
  Purchase #2 $17.37   13.179%   $19.6592   $19.66   $19.65   $19.70  
  Purchase #3 $93.38   9.770%   $102.5032   $102.50   $102.50   $102.50  
  Purchase #4 $47.94   7.691%   $51.6271   $51.63   $51.65   $51.60  
  Purchase #5 $104.46   4.749%   $109.4208   $109.42   $109.40   $109.40  
  Purchase #6 $132.53   11.810%   $148.1818   $148.18   $148.20   $148.20  
  Purchase #7 $128.63   12.557%   $144.7821   $144.78   $144.80   $144.80  
  7 purchase totals = $544.16       $597.3121   $597.31   $597.35   $597.30