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 $46.36   9.431%   $50.7322   $50.73   $50.75   $50.70  
  Purchase #2 $39.52   6.771%   $42.1959   $42.20   $42.20   $42.20  
  Purchase #3 $5.73   13.397%   $6.4976   $6.50   $6.50   $6.50  
  Purchase #4 $242.57   6.826%   $259.1278   $259.13   $259.15   $259.10  
  Purchase #5 $108.90   5.388%   $114.7675   $114.77   $114.75   $114.80  
  Purchase #6 $141.52   11.383%   $157.6292   $157.63   $157.65   $157.60  
  Purchase #7 $8.66   4.684%   $9.0656   $9.07   $9.05   $9.10  
  7 purchase totals = $593.26       $640.0158   $640.03   $640.05   $640.00