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 $67.75   3.221%   $69.9322   $69.93   $69.95   $69.90  
  Purchase #2 $81.57   6.194%   $86.6224   $86.62   $86.60   $86.60  
  Purchase #3 $110.68   9.706%   $121.4226   $121.42   $121.40   $121.40  
  Purchase #4 $191.55   8.599%   $208.0214   $208.02   $208.00   $208.00  
  Purchase #5 $68.89   9.437%   $75.3911   $75.39   $75.40   $75.40  
  Purchase #6 $209.42   3.819%   $217.4177   $217.42   $217.40   $217.40  
  Purchase #7 $74.69   5.537%   $78.8256   $78.83   $78.85   $78.80  
  7 purchase totals = $804.55       $857.6330   $857.63   $857.60   $857.50