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 $139.99   6.712%   $149.3861   $149.39   $149.40   $149.40  
  Purchase #2 $12.62   13.549%   $14.3299   $14.33   $14.35   $14.30  
  Purchase #3 $198.86   11.541%   $221.8104   $221.81   $221.80   $221.80  
  Purchase #4 $83.84   4.650%   $87.7386   $87.74   $87.75   $87.70  
  Purchase #5 $33.61   6.704%   $35.8632   $35.86   $35.85   $35.90  
  Purchase #6 $79.34   10.182%   $87.4184   $87.42   $87.40   $87.40  
  Purchase #7 $54.81   12.175%   $61.4831   $61.48   $61.50   $61.50  
  7 purchase totals = $603.07       $658.0297   $658.03   $658.05   $658.00