http://quaketips.blogspot.com/2013/05/should-i-turn-my-gas-off-after.htmlThis and other sources suggest that turning off the gas should not be done in a knee jerk fashion. It may be a while before you can get the utility company to turn it back on. On the other hand, if you turn the gas off, it is indeed off.
The decision will vary with the circumstances. I would be just as aware of downed electric wires, among the many potential haards, as leaking gas.
I agree. Whether or not to turn it off after an earthquake depends on the circumstances.
I would certainly turn it off if I detected any hint of leaking gas. I would also likely turn it off if there were indications of significant structural damage to the house. On the other hand, absent any indication of leaking gas, and if there were no serious structural damage, I would probably leave it on.
By the way, it is possible to turn it back on yourself, at least for the set up at my house. The gas guy showed me how to do it (though he probably wasn't supposed to). It was actually quite simple, at least for my place.