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 $234.92   13.369%   $266.3265   $266.33   $266.35   $266.30  
  Purchase #2 $65.38   13.654%   $74.307   $74.31   $74.30   $74.30  
  Purchase #3 $66.32   3.553%   $68.6763   $68.68   $68.70   $68.70  
  Purchase #4 $13.88   11.615%   $15.4922   $15.49   $15.50   $15.50  
  Purchase #5 $176.41   9.189%   $192.6203   $192.62   $192.60   $192.60  
  Purchase #6 $27.97   12.575%   $31.4872   $31.49   $31.50   $31.50  
  Purchase #7 $236.44   3.521%   $244.7651   $244.77   $244.75   $244.80  
  7 purchase totals = $821.32       $893.6746   $893.69   $893.70   $893.70