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 $27.40   5.583%   $28.9297   $28.93   $28.95   $28.90  
  Purchase #2 $30.34   11.771%   $33.9113   $33.91   $33.90   $33.90  
  Purchase #3 $130.35   6.813%   $139.2307   $139.23   $139.25   $139.20  
  Purchase #4 $24.27   11.431%   $27.0443   $27.04   $27.05   $27.00  
  Purchase #5 $178.77   11.849%   $199.9525   $199.95   $199.95   $200.00  
  Purchase #6 $7.70   9.376%   $8.422   $8.42   $8.40   $8.40  
  Purchase #7 $136.46   8.508%   $148.07   $148.07   $148.05   $148.10  
  7 purchase totals = $535.29       $585.5605   $585.55   $585.55   $585.50