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 $44.59   13.228%   $50.4884   $50.49   $50.50   $50.50  
  Purchase #2 $15.58   3.874%   $16.1836   $16.18   $16.20   $16.20  
  Purchase #3 $194.47   13.411%   $220.5504   $220.55   $220.55   $220.60  
  Purchase #4 $246.25   3.623%   $255.1716   $255.17   $255.15   $255.20  
  Purchase #5 $190.75   8.149%   $206.2942   $206.29   $206.30   $206.30  
  Purchase #6 $162.58   7.865%   $175.3669   $175.37   $175.35   $175.40  
  Purchase #7 $65.43   11.601%   $73.0205   $73.02   $73.00   $73.00  
  7 purchase totals = $919.65       $997.0756   $997.07   $997.05   $997.20