As there are so many I usually just place explosion barrel right next to a flame can near critical location so you are sure to hit atleast one of them when shooting that direction.
Other ones are placed around possible escape routes or places where you might need to get ammo and need fast way to get rid of zombies hitting you (after all burning survivor isn't necessarily dead survivor)
Good way to get them used might be to place them near your first stand off ground, then move on and set the place on fire when you run out of ammo etc.
Tried to use against tanks or save them for later, but they tend to get shot at when zombies start swarming