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 $9.94   9.774%   $10.9115   $10.91   $10.90   $10.90  
  Purchase #2 $16.56   5.662%   $17.4976   $17.50   $17.50   $17.50  
  Purchase #3 $183.59   7.400%   $197.1757   $197.18   $197.20   $197.20  
  Purchase #4 $56.54   8.665%   $61.4392   $61.44   $61.45   $61.40  
  Purchase #5 $139.32   10.533%   $153.9946   $153.99   $154.00   $154.00  
  Purchase #6 $10.95   11.458%   $12.2047   $12.20   $12.20   $12.20  
  Purchase #7 $167.74   10.681%   $185.6563   $185.66   $185.65   $185.70  
  7 purchase totals = $584.64       $638.8796   $638.88   $638.90   $638.90