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 $28.90   5.333%   $30.4412   $30.44   $30.45   $30.40  
  Purchase #2 $164.80   3.298%   $170.2351   $170.24   $170.25   $170.20  
  Purchase #3 $19.43   3.786%   $20.1656   $20.17   $20.15   $20.20  
  Purchase #4 $237.88   8.792%   $258.7944   $258.79   $258.80   $258.80  
  Purchase #5 $129.67   8.648%   $140.8839   $140.88   $140.90   $140.90  
  Purchase #6 $231.39   12.675%   $260.7187   $260.72   $260.70   $260.70  
  Purchase #7 $59.30   6.442%   $63.1201   $63.12   $63.10   $63.10  
  7 purchase totals = $871.37       $944.3590   $944.36   $944.35   $944.30