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 $243.90   11.147%   $271.0875   $271.09   $271.10   $271.10  
  Purchase #2 $161.95   13.221%   $183.3614   $183.36   $183.35   $183.40  
  Purchase #3 $92.40   9.810%   $101.4644   $101.46   $101.45   $101.50  
  Purchase #4 $194.53   8.688%   $211.4308   $211.43   $211.45   $211.40  
  Purchase #5 $244.88   5.124%   $257.4277   $257.43   $257.45   $257.40  
  Purchase #6 $12.38   4.202%   $12.9002   $12.90   $12.90   $12.90  
  Purchase #7 $199.83   4.426%   $208.6745   $208.67   $208.65   $208.70  
  7 purchase totals = $1,149.87       $1,246.3465   $1,246.34   $1,246.35   $1,246.40