[47hook]

Actually, with the flats that I've had, the rim is sitting on the flattened tire which is still in contact with the ground which is then still supporting weight. The concern I would have is that if the tire pressure goes below that which is needed to support the weight that the tire is supporting, you have overloaded that tire. At Goodyear's website, it states that a tire that has lost 20% of it's pressure (IIRC) is considered flat and should be inspected for damage, not just what caused it to go flat. Plus, it can't be good for a tire to be squashed flat if completely or almost completely out of air.

So, if you have a slow leaker, try not to let it go below the pressure needed to support the weight.

So far as jacking, I agree with those that say use the spring perch as a jacking point, provided you don't raise the axle past the point it would be with a fully inflated tire. Don't think that would put any different stress than being supported by a tire.