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 $227.44   13.227%   $257.5235   $257.52   $257.50   $257.50  
  Purchase #2 $75.70   3.860%   $78.622   $78.62   $78.60   $78.60  
  Purchase #3 $52.57   5.372%   $55.3941   $55.39   $55.40   $55.40  
  Purchase #4 $247.97   6.750%   $264.708   $264.71   $264.70   $264.70  
  Purchase #5 $34.38   3.774%   $35.6775   $35.68   $35.70   $35.70  
  Purchase #6 $23.38   12.675%   $26.3434   $26.34   $26.35   $26.30  
  Purchase #7 $74.77   3.395%   $77.3084   $77.31   $77.30   $77.30  
  7 purchase totals = $736.21       $795.5769   $795.57   $795.55   $795.50