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 $134.53   3.589%   $139.3583   $139.36   $139.35   $139.40  
  Purchase #2 $122.48   12.749%   $138.095   $138.10   $138.10   $138.10  
  Purchase #3 $37.87   7.559%   $40.7326   $40.73   $40.75   $40.70  
  Purchase #4 $39.85   5.807%   $42.1641   $42.16   $42.15   $42.20  
  Purchase #5 $97.52   4.360%   $101.7719   $101.77   $101.75   $101.80  
  Purchase #6 $78.48   11.326%   $87.3686   $87.37   $87.35   $87.40  
  Purchase #7 $28.99   3.252%   $29.9328   $29.93   $29.95   $29.90  
  7 purchase totals = $539.72       $579.4233   $579.42   $579.40   $579.50