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 $12.72   8.590%   $13.8126   $13.81   $13.80   $13.80  
  Purchase #2 $116.71   10.508%   $128.9739   $128.97   $128.95   $129.00  
  Purchase #3 $10.51   4.153%   $10.9465   $10.95   $10.95   $10.90  
  Purchase #4 $46.80   13.425%   $53.0829   $53.08   $53.10   $53.10  
  Purchase #5 $139.25   12.777%   $157.042   $157.04   $157.05   $157.00  
  Purchase #6 $123.61   5.601%   $130.5334   $130.53   $130.55   $130.50  
  Purchase #7 $100.83   3.114%   $103.9698   $103.97   $103.95   $104.00  
  7 purchase totals = $550.43       $598.3611   $598.35   $598.35   $598.30