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 $239.64   12.237%   $268.9647   $268.96   $268.95   $269.00  
  Purchase #2 $85.55   7.141%   $91.6591   $91.66   $91.65   $91.70  
  Purchase #3 $28.37   4.275%   $29.5828   $29.58   $29.60   $29.60  
  Purchase #4 $82.61   11.382%   $92.0127   $92.01   $92.00   $92.00  
  Purchase #5 $199.71   3.441%   $206.582   $206.58   $206.60   $206.60  
  Purchase #6 $56.36   6.707%   $60.1401   $60.14   $60.15   $60.10  
  Purchase #7 $77.63   11.845%   $86.8253   $86.83   $86.85   $86.80  
  7 purchase totals = $769.87       $835.7667   $835.76   $835.80   $835.80