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 $230.39   13.276%   $260.9766   $260.98   $261.00   $261.00  
  Purchase #2 $63.33   9.848%   $69.5667   $69.57   $69.55   $69.60  
  Purchase #3 $9.95   12.872%   $11.2308   $11.23   $11.25   $11.20  
  Purchase #4 $41.69   10.431%   $46.0387   $46.04   $46.05   $46.00  
  Purchase #5 $125.50   8.686%   $136.4009   $136.40   $136.40   $136.40  
  Purchase #6 $186.84   11.724%   $208.7451   $208.75   $208.75   $208.70  
  Purchase #7 $200.40   12.761%   $225.973   $225.97   $225.95   $226.00  
  7 purchase totals = $858.10       $958.9318   $958.94   $958.95   $958.90