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 $40.63   4.364%   $42.4031   $42.40   $42.40   $42.40  
  Purchase #2 $169.49   4.684%   $177.4289   $177.43   $177.45   $177.40  
  Purchase #3 $197.70   9.500%   $216.4815   $216.48   $216.50   $216.50  
  Purchase #4 $79.25   12.340%   $89.0295   $89.03   $89.05   $89.00  
  Purchase #5 $20.98   9.134%   $22.8963   $22.90   $22.90   $22.90  
  Purchase #6 $19.44   10.769%   $21.5335   $21.53   $21.55   $21.50  
  Purchase #7 $78.86   5.428%   $83.1405   $83.14   $83.15   $83.10  
  7 purchase totals = $606.35       $652.9133   $652.91   $653.00   $652.80