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 $249.71   8.809%   $271.707   $271.71   $271.70   $271.70  
  Purchase #2 $62.63   11.639%   $69.9195   $69.92   $69.90   $69.90  
  Purchase #3 $75.96   5.586%   $80.2031   $80.20   $80.20   $80.20  
  Purchase #4 $51.95   9.120%   $56.6878   $56.69   $56.70   $56.70  
  Purchase #5 $123.90   8.404%   $134.3126   $134.31   $134.30   $134.30  
  Purchase #6 $214.32   8.500%   $232.5372   $232.54   $232.55   $232.50  
  Purchase #7 $9.71   10.136%   $10.6942   $10.69   $10.70   $10.70  
  7 purchase totals = $788.18       $856.0614   $856.06   $856.05   $856.00