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I think part of the illegal aspect of shifting into neutral has more to do with vehicles from the era before transmissions had synchros than anything else. There was a school bus that infamously lost its brakes and went off the side of a steep mountain pass killing many of the children because the driver had shifted into neutral and couldn't get it back into gear. The same can be said of if you need to accelerate. So even if you don't need any sort of braking for a certain grade, if it's a bear to get back into gear to accelerate you could have a safety problem.
However, I'm not going to recommend braking the law, but I don't see how shifting into neutral with a manual transmission in a modern car that you can shift back into gear in a fraction of a second is going to cause a safety concern, as long as you aren't riding your brakes as a result. If you have a hard time getting it into gear, with or wihtout synchromesh, then it's probably better to just leave it in gear for safety.
As far as transmission wear, there are some modern manual transmissions that do have a lubrication pump that only works if the tranmission is engauged. They are rare, I think mainly on bigger vehicles, but it's something to be sure of before coasting in neutral (or towing your vehicle in neutral).
Double clutching (blipping the throttle before stepping on the clutch and shifting into gear) is definitely a good idea in my opinion. It may wear out your clutch linkage a bit quicker, but helps save the transmission and even the cluch itself. Replacing a cable or clutch cylinder is a lot easier than replacing a clutch and/or flywheel or rebuilding a transmission. I double clutch anytime I'm downshifting, regardless of the vehicle (well, with a few exceptions honestly.)
Although if you're idling down a hill you're still burining fuel. Shutting off the engine would burn zero fuel, but then you can't blip the throttle to get the gears to mesh.
Of course if you need to brake at all you're better off leaving it in gear. If you think you're still burning fuel, you could install some sort of fuel shut off and get even more engine braking. Of course if you need to down shift again you would be able to blip the throttle without turnin the fuel back on again.
In my Super Beetle I had fun shutting off the ignition, which also closes the idle shut off valve. Then I'd turn the ignition back on. The high vacuum of engine braking would keep the idle shut off valve closed. But once I got to the bottom, all I had to do was touch the throttle or shift into a lower gear and the vacuum would drop enough to open the idle shut off valve again and away I'd go.
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