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 $17.89   5.117%   $18.8054   $18.81   $18.80   $18.80  
  Purchase #2 $121.28   9.475%   $132.7713   $132.77   $132.75   $132.80  
  Purchase #3 $123.63   8.118%   $133.6663   $133.67   $133.65   $133.70  
  Purchase #4 $25.54   10.375%   $28.1898   $28.19   $28.20   $28.20  
  Purchase #5 $144.61   3.859%   $150.1905   $150.19   $150.20   $150.20  
  Purchase #6 $72.68   7.378%   $78.0423   $78.04   $78.05   $78.00  
  Purchase #7 $215.68   7.365%   $231.5648   $231.56   $231.55   $231.60  
  7 purchase totals = $721.31       $773.2304   $773.23   $773.20   $773.30