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 $239.52   8.187%   $259.1295   $259.13   $259.15   $259.10  
  Purchase #2 $104.44   10.757%   $115.6746   $115.67   $115.65   $115.70  
  Purchase #3 $159.46   7.478%   $171.3844   $171.38   $171.40   $171.40  
  Purchase #4 $223.36   10.349%   $246.4755   $246.48   $246.50   $246.50  
  Purchase #5 $208.36   11.585%   $232.4985   $232.50   $232.50   $232.50  
  Purchase #6 $153.98   11.754%   $172.0788   $172.08   $172.10   $172.10  
  Purchase #7 $71.53   8.240%   $77.4241   $77.42   $77.40   $77.40  
  7 purchase totals = $1,160.65       $1,274.6654   $1,274.66   $1,274.70   $1,274.70