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 $78.46   6.777%   $83.7772   $83.78   $83.80   $83.80  
  Purchase #2 $21.32   3.305%   $22.0246   $22.02   $22.00   $22.00  
  Purchase #3 $237.31   11.859%   $265.4526   $265.45   $265.45   $265.50  
  Purchase #4 $73.97   11.108%   $82.1866   $82.19   $82.20   $82.20  
  Purchase #5 $185.92   4.361%   $194.028   $194.03   $194.05   $194.00  
  Purchase #6 $28.49   7.293%   $30.5678   $30.57   $30.55   $30.60  
  Purchase #7 $15.43   3.134%   $15.9136   $15.91   $15.90   $15.90  
  7 purchase totals = $640.90       $693.9504   $693.95   $693.95   $694.00