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 $28.32   13.231%   $32.067   $32.07   $32.05   $32.10  
  Purchase #2 $231.58   12.841%   $261.3172   $261.32   $261.30   $261.30  
  Purchase #3 $22.44   6.700%   $23.9435   $23.94   $23.95   $23.90  
  Purchase #4 $250.85   4.141%   $261.2377   $261.24   $261.25   $261.20  
  Purchase #5 $35.49   4.372%   $37.0416   $37.04   $37.05   $37.00  
  Purchase #6 $127.38   12.710%   $143.57   $143.57   $143.55   $143.60  
  Purchase #7 $68.86   8.634%   $74.8054   $74.81   $74.80   $74.80  
  7 purchase totals = $764.92       $833.9824   $833.99   $833.95   $833.90