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 $239.54   9.319%   $261.8627   $261.86   $261.85   $261.90  
  Purchase #2 $23.75   12.689%   $26.7636   $26.76   $26.75   $26.80  
  Purchase #3 $50.31   11.703%   $56.1978   $56.20   $56.20   $56.20  
  Purchase #4 $93.86   6.770%   $100.2143   $100.21   $100.20   $100.20  
  Purchase #5 $130.64   5.678%   $138.0577   $138.06   $138.05   $138.10  
  Purchase #6 $154.38   12.807%   $174.1514   $174.15   $174.15   $174.20  
  Purchase #7 $180.97   10.413%   $199.8144   $199.81   $199.80   $199.80  
  7 purchase totals = $873.45       $957.0619   $957.05   $957.00   $957.20