A rule of thumb which has worked for the types I’ve flown, is that for each thousand feet you are away from the optimum altitude it costs 2% on the fuel burn.
If you are held off optimum by ATC, you may have to sacrifice a bit of speed, in an attempt to limit the damage to your fuel figures. It doesn’t matter all that much on short flights (anything less than about 5 hours), but once into long haul, a 2% penalty would eat all of your variable reserve, and could easily make the difference between going to destination, or elsewhere.
From a tactical point of view, if ATC offer a choice of higher or lower than optimum, it’s generally better to take the higher. Eventually optimum will climb up to the altitude you’re at, whereas lower, the differential gets worse. Out of LA the 380 is generally able to get that little bit higher than either the 787s or the 777s, so you’re normally not blocked by anything, other than another 380. Leaving Perth the 787 won’t have many issues with altitude, but it may have some out of London.