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 $224.53   11.152%   $249.5696   $249.57   $249.55   $249.60  
  Purchase #2 $164.52   7.316%   $176.5563   $176.56   $176.55   $176.60  
  Purchase #3 $45.85   11.763%   $51.2433   $51.24   $51.25   $51.20  
  Purchase #4 $171.61   4.757%   $179.7735   $179.77   $179.75   $179.80  
  Purchase #5 $76.43   10.189%   $84.2175   $84.22   $84.20   $84.20  
  Purchase #6 $237.76   6.604%   $253.4617   $253.46   $253.45   $253.50  
  Purchase #7 $243.57   10.434%   $268.9841   $268.98   $269.00   $269.00  
  7 purchase totals = $1,164.27       $1,263.8060   $1,263.80   $1,263.75   $1,263.90