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 $36.53   10.597%   $40.4011   $40.40   $40.40   $40.40  
  Purchase #2 $217.80   12.571%   $245.1796   $245.18   $245.20   $245.20  
  Purchase #3 $229.84   4.706%   $240.6563   $240.66   $240.65   $240.70  
  Purchase #4 $37.52   6.781%   $40.0642   $40.06   $40.05   $40.10  
  Purchase #5 $28.57   11.833%   $31.9507   $31.95   $31.95   $32.00  
  Purchase #6 $17.66   4.487%   $18.4524   $18.45   $18.45   $18.50  
  Purchase #7 $26.79   4.211%   $27.9181   $27.92   $27.90   $27.90  
  7 purchase totals = $594.71       $644.6224   $644.62   $644.60   $644.80