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 $18.62   8.314%   $20.1681   $20.17   $20.15   $20.20  
  Purchase #2 $70.87   6.304%   $75.3376   $75.34   $75.35   $75.30  
  Purchase #3 $40.56   8.580%   $44.04   $44.04   $44.05   $44.00  
  Purchase #4 $161.46   3.688%   $167.4146   $167.41   $167.40   $167.40  
  Purchase #5 $158.43   10.524%   $175.1032   $175.10   $175.10   $175.10  
  Purchase #6 $15.83   12.493%   $17.8076   $17.81   $17.80   $17.80  
  Purchase #7 $17.44   12.557%   $19.6299   $19.63   $19.65   $19.60  
  7 purchase totals = $483.21       $519.5010   $519.50   $519.50   $519.40