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 $61.63   6.256%   $65.4856   $65.49   $65.50   $65.50  
  Purchase #2 $57.77   7.471%   $62.086   $62.09   $62.10   $62.10  
  Purchase #3 $11.71   12.171%   $13.1352   $13.14   $13.15   $13.10  
  Purchase #4 $79.65   8.293%   $86.2554   $86.26   $86.25   $86.30  
  Purchase #5 $186.46   12.592%   $209.939   $209.94   $209.95   $209.90  
  Purchase #6 $129.87   10.821%   $143.9232   $143.92   $143.90   $143.90  
  Purchase #7 $30.57   5.197%   $32.1587   $32.16   $32.15   $32.20  
  7 purchase totals = $557.66       $612.9831   $613.00   $613.00   $613.00