They'll burn as much fuel as necessary to complete the mission. But the more fuel you burn just getting into the air, the less you have to complete the mission. You might leave ordnance behind and have enough fuel to get to your target and back, but then you have a smaller payload to drop on the enemy. This is somewhat alleviated with in-air refueling. However insufficient fuel may be a factor in a tactical situation. My ex-fiancée was a Black Hawk pilot during the Gulf War. On one mission they landed with 60 pounds of fuel. (Or maybe less; I don't remember.) You can never have too much fuel -- unless you're on fire.