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 $231.35   6.334%   $246.0037   $246.00   $246.00   $246.00  
  Purchase #2 $21.65   6.248%   $23.0027   $23.00   $23.00   $23.00  
  Purchase #3 $169.44   4.551%   $177.1512   $177.15   $177.15   $177.20  
  Purchase #4 $19.36   4.464%   $20.2242   $20.22   $20.20   $20.20  
  Purchase #5 $10.70   11.554%   $11.9363   $11.94   $11.95   $11.90  
  Purchase #6 $168.33   5.641%   $177.8255   $177.83   $177.85   $177.80  
  Purchase #7 $119.56   4.267%   $124.6616   $124.66   $124.65   $124.70  
  7 purchase totals = $740.39       $780.8052   $780.80   $780.80   $780.80