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 $12.39   4.246%   $12.9161   $12.92   $12.90   $12.90  
  Purchase #2 $88.54   12.335%   $99.4614   $99.46   $99.45   $99.50  
  Purchase #3 $214.54   11.725%   $239.6948   $239.69   $239.70   $239.70  
  Purchase #4 $79.84   8.832%   $86.8915   $86.89   $86.90   $86.90  
  Purchase #5 $42.68   11.104%   $47.4192   $47.42   $47.40   $47.40  
  Purchase #6 $9.71   9.415%   $10.6242   $10.62   $10.60   $10.60  
  Purchase #7 $49.30   12.262%   $55.3452   $55.35   $55.35   $55.30  
  7 purchase totals = $497.00       $552.3524   $552.35   $552.30   $552.30