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 $216.77   10.699%   $239.9622   $239.96   $239.95   $240.00  
  Purchase #2 $84.73   10.545%   $93.6648   $93.66   $93.65   $93.70  
  Purchase #3 $196.37   11.241%   $218.444   $218.44   $218.45   $218.40  
  Purchase #4 $5.43   13.431%   $6.1593   $6.16   $6.15   $6.20  
  Purchase #5 $97.68   5.689%   $103.237   $103.24   $103.25   $103.20  
  Purchase #6 $249.95   8.776%   $271.8856   $271.89   $271.90   $271.90  
  Purchase #7 $91.89   9.656%   $100.7629   $100.76   $100.75   $100.80  
  7 purchase totals = $942.82       $1,034.1158   $1,034.11   $1,034.10   $1,034.20