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 $139.49   5.126%   $146.6403   $146.64   $146.65   $146.60  
  Purchase #2 $217.98   11.313%   $242.6401   $242.64   $242.65   $242.60  
  Purchase #3 $154.87   3.162%   $159.767   $159.77   $159.75   $159.80  
  Purchase #4 $148.63   12.664%   $167.4525   $167.45   $167.45   $167.50  
  Purchase #5 $141.77   6.865%   $151.5025   $151.50   $151.50   $151.50  
  Purchase #6 $151.36   10.332%   $166.9985   $167.00   $167.00   $167.00  
  Purchase #7 $175.94   6.123%   $186.7128   $186.71   $186.70   $186.70  
  7 purchase totals = $1,130.04       $1,221.7137   $1,221.71   $1,221.70   $1,221.70