Juan Brown was talking about this relative to the latest snow storms out where he lives (4 feet of snow). He noted planes tied down at his local airport collecting enough snow on the wings and tail to exceed their negative G limits, made worse by how tight the wing tie-downs were...pushing down the tail from snow weight put even bigger lever loads on the wings that were tightly tied down. He predicts several of those planes will have been over-G'd from the snow load. Granted, not a consideration for most of us, but interesting to think about.
Knotting the tiedowns in the venerable old half-hitch manner is problematic with the Dyneema rope, for example, which wants to untie itself under load (not to mention less convenient). I'm looking around for ratcheting rope hooks like the NiteIze CamJam, but something stronger. In the meantime, I've been using some cam lock tie down straps and carry some soft loops for those occasional problematic recessed concrete anchors.