LED's for all practical purposes are pretty close to being physically indestructible. The smaller 5mm ones are a diode encased in a solid chunk of plastic, nothing short of a hammer blow directly on it can break it. The higher power ones are designed a bit differently, but as long a they're contained within a flashlight head I can't see how one could be destroyed under normal use. They are relatively impervious to shock and vibration, the main causes of failures for incandescents.

However, while the LED is physically durable, they're more sensitive to other forms of failure such as thermal or current overload. This is why you might see some LED failures on the cheaper multi-led flashlights, because the LED's are not being powered equally. Also, the electronics needed to drive the them may not be as reliable as the LED's themselves, so that's another consideration

I personally wouldn't bother stocking spare bulbs for and LED flashlight, there really isn't a need. The technology advances so quickly that by the time you need it (if ever) it'll probably be obsolete. I'd carry a complete flashlight as a spare instead, because if your flashlight does fail, chances are it's probably something other than the LED. That's the thing about failure modes, it will always find the weakest link. With the invention of the LED's, the bulb is no longer the weakest link, it's probably the batteries themselves. Back when LED's first started becoming mainstream, people were subjecting them to ridiculous torture testing like throwing them off buildings. The LED's didn't fail, but the batteries would be dented so badly that they wouldn't make the connection inside the flashlight anymore