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 $19.27   9.466%   $21.0941   $21.09   $21.10   $21.10  
  Purchase #2 $126.49   10.587%   $139.8815   $139.88   $139.90   $139.90  
  Purchase #3 $50.91   5.548%   $53.7345   $53.73   $53.75   $53.70  
  Purchase #4 $199.64   5.295%   $210.2109   $210.21   $210.20   $210.20  
  Purchase #5 $56.86   8.861%   $61.8984   $61.90   $61.90   $61.90  
  Purchase #6 $172.89   11.362%   $192.5338   $192.53   $192.55   $192.50  
  Purchase #7 $55.42   9.284%   $60.5652   $60.57   $60.55   $60.60  
  7 purchase totals = $681.48       $739.9184   $739.91   $739.95   $739.90