The strength of a knot is by design, some are stronger than others. Security of a knot is determined by the way the end of the rope is finished or locked in the knot. Secure knots as necessary and not as secure as possible. The US FAA has published AC 20-35C Tiedown Sense, with tips on aircraft tiedowns.
Below you will find part two, for part one click here. But remember: the simplest and easiest thing to do is fly you aircraft out of the storm area or park it inside a storm proof hanger.
Part two, pages 11 - 20, for tips for tying down aircraft below.
Excerpt from the document:
Securing aircraft. Tie only at the tiedown rings provided for that purpose. Never tie to a strut itself. The practice of tying to lift struts has in itself caused frequent damage. Ropes slip to a point when even slight pressure may bend the struts.
Allow for about 1 inch of movement, and remember that manila rope shrinks when it gets wet. TOO much slack will allow the aircraft to jerk against the ropes. Avoid tightening the ropes too much. Tight tiedown ropes actually put inverted flight stresses on the aircraft, and many of them are not designed to take such loads.
To continue reading see the pdf below: