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 $246.84   4.117%   $257.0024   $257.00   $257.00   $257.00  
  Purchase #2 $118.62   11.660%   $132.4511   $132.45   $132.45   $132.50  
  Purchase #3 $134.99   8.138%   $145.9755   $145.98   $146.00   $146.00  
  Purchase #4 $78.67   5.762%   $83.203   $83.20   $83.20   $83.20  
  Purchase #5 $161.30   11.869%   $180.4447   $180.44   $180.45   $180.40  
  Purchase #6 $107.66   3.704%   $111.6477   $111.65   $111.65   $111.60  
  Purchase #7 $89.54   4.423%   $93.5004   $93.50   $93.50   $93.50  
  7 purchase totals = $937.62       $1,004.2248   $1,004.22   $1,004.25   $1,004.20