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 $212.63   12.560%   $239.3363   $239.34   $239.35   $239.30  
  Purchase #2 $126.48   13.631%   $143.7205   $143.72   $143.70   $143.70  
  Purchase #3 $18.37   11.318%   $20.4491   $20.45   $20.45   $20.40  
  Purchase #4 $147.38   10.703%   $163.1541   $163.15   $163.15   $163.20  
  Purchase #5 $148.27   3.830%   $153.9487   $153.95   $153.95   $153.90  
  Purchase #6 $182.46   8.372%   $197.7356   $197.74   $197.75   $197.70  
  Purchase #7 $72.84   5.159%   $76.5978   $76.60   $76.60   $76.60  
  7 purchase totals = $908.43       $994.9421   $994.95   $994.95   $994.80