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 $18.66   13.323%   $21.1461   $21.15   $21.15   $21.10  
  Purchase #2 $51.73   10.542%   $57.1834   $57.18   $57.20   $57.20  
  Purchase #3 $247.77   7.511%   $266.38   $266.38   $266.40   $266.40  
  Purchase #4 $37.46   6.755%   $39.9904   $39.99   $40.00   $40.00  
  Purchase #5 $84.92   5.538%   $89.6229   $89.62   $89.60   $89.60  
  Purchase #6 $95.59   4.350%   $99.7482   $99.75   $99.75   $99.70  
  Purchase #7 $79.57   5.853%   $84.2272   $84.23   $84.25   $84.20  
  7 purchase totals = $615.70       $658.2982   $658.30   $658.35   $658.20