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 $60.83   8.783%   $66.1727   $66.17   $66.15   $66.20  
  Purchase #2 $214.62   11.804%   $239.9537   $239.95   $239.95   $240.00  
  Purchase #3 $6.42   8.498%   $6.9656   $6.97   $6.95   $7.00  
  Purchase #4 $223.75   11.502%   $249.4857   $249.49   $249.50   $249.50  
  Purchase #5 $120.72   9.753%   $132.4938   $132.49   $132.50   $132.50  
  Purchase #6 $188.38   10.446%   $208.0582   $208.06   $208.05   $208.10  
  Purchase #7 $85.65   3.667%   $88.7908   $88.79   $88.80   $88.80  
  7 purchase totals = $900.37       $991.9205   $991.92   $991.90   $992.10