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 $248.75   5.236%   $261.7746   $261.77   $261.75   $261.80  
  Purchase #2 $115.64   3.681%   $119.8967   $119.90   $119.90   $119.90  
  Purchase #3 $10.69   5.654%   $11.2944   $11.29   $11.30   $11.30  
  Purchase #4 $151.37   3.778%   $157.0888   $157.09   $157.10   $157.10  
  Purchase #5 $221.52   8.211%   $239.709   $239.71   $239.70   $239.70  
  Purchase #6 $250.86   12.252%   $281.5954   $281.60   $281.60   $281.60  
  Purchase #7 $136.25   8.217%   $147.4457   $147.45   $147.45   $147.40  
  7 purchase totals = $1,135.08       $1,218.8046   $1,218.81   $1,218.80   $1,218.80