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 $19.83   5.361%   $20.8931   $20.89   $20.90   $20.90  
  Purchase #2 $9.27   5.559%   $9.7853   $9.79   $9.80   $9.80  
  Purchase #3 $139.30   12.836%   $157.1805   $157.18   $157.20   $157.20  
  Purchase #4 $24.71   12.698%   $27.8477   $27.85   $27.85   $27.80  
  Purchase #5 $154.59   7.350%   $165.9524   $165.95   $165.95   $166.00  
  Purchase #6 $108.82   7.604%   $117.0947   $117.09   $117.10   $117.10  
  Purchase #7 $157.30   3.254%   $162.4185   $162.42   $162.40   $162.40  
  7 purchase totals = $613.82       $661.1722   $661.17   $661.20   $661.20