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 $56.41   6.422%   $60.0327   $60.03   $60.05   $60.00  
  Purchase #2 $24.93   5.604%   $26.3271   $26.33   $26.35   $26.30  
  Purchase #3 $228.90   3.162%   $236.1378   $236.14   $236.15   $236.10  
  Purchase #4 $151.71   10.574%   $167.7518   $167.75   $167.75   $167.80  
  Purchase #5 $100.73   3.500%   $104.2556   $104.26   $104.25   $104.30  
  Purchase #6 $177.89   4.754%   $186.3469   $186.35   $186.35   $186.30  
  Purchase #7 $70.45   5.722%   $74.4811   $74.48   $74.50   $74.50  
  7 purchase totals = $811.02       $855.3330   $855.34   $855.40   $855.30