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 $52.49   13.226%   $59.4323   $59.43   $59.45   $59.40  
  Purchase #2 $248.29   9.849%   $272.7441   $272.74   $272.75   $272.70  
  Purchase #3 $58.72   8.653%   $63.801   $63.80   $63.80   $63.80  
  Purchase #4 $103.61   4.770%   $108.5522   $108.55   $108.55   $108.60  
  Purchase #5 $214.89   4.618%   $224.8136   $224.81   $224.80   $224.80  
  Purchase #6 $221.55   5.734%   $234.2537   $234.25   $234.25   $234.30  
  Purchase #7 $15.71   10.389%   $17.3421   $17.34   $17.35   $17.30  
  7 purchase totals = $915.26       $980.9390   $980.92   $980.95   $980.90