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 $19.99   7.538%   $21.4968   $21.50   $21.50   $21.50  
  Purchase #2 $108.67   4.210%   $113.245   $113.25   $113.25   $113.20  
  Purchase #3 $208.91   3.728%   $216.6982   $216.70   $216.70   $216.70  
  Purchase #4 $152.26   11.410%   $169.6329   $169.63   $169.65   $169.60  
  Purchase #5 $121.52   4.728%   $127.2655   $127.27   $127.25   $127.30  
  Purchase #6 $124.26   5.551%   $131.1577   $131.16   $131.15   $131.20  
  Purchase #7 $32.80   10.546%   $36.2591   $36.26   $36.25   $36.30  
  7 purchase totals = $768.41       $815.7552   $815.77   $815.75   $815.80