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 $203.96   6.657%   $217.5376   $217.54   $217.55   $217.50  
  Purchase #2 $111.32   7.552%   $119.7269   $119.73   $119.75   $119.70  
  Purchase #3 $9.37   6.756%   $10.003   $10.00   $10.00   $10.00  
  Purchase #4 $59.87   5.585%   $63.2137   $63.21   $63.20   $63.20  
  Purchase #5 $236.32   12.211%   $265.177   $265.18   $265.20   $265.20  
  Purchase #6 $50.99   8.803%   $55.4786   $55.48   $55.50   $55.50  
  Purchase #7 $233.56   11.282%   $259.9102   $259.91   $259.90   $259.90  
  7 purchase totals = $905.39       $991.0470   $991.05   $991.10   $991.00