Said another way, you should always be aware of the direction of your muzzle, and you should always keep that muzzle pointed in a safe direction.
Since your firearm has to point somewhere, you should always ensure that it's pointed in a direction that can serve as a backstop, and will not cause a dangerous ricochet if the firearm where to discharge. That criteria would indicate that pointing the firearm straight up or straight down would not be considered a safe direction, since a round negligently fired up may penetrate a ceiling; ricochet off the ceiling; or travel for thousands of feet if fired outside; and a round fired starlight down can penetrate a floor or ricochet off the floor or ground.
A good method to practice this rule is to pretend that a laser extends out from the end of the barrel. You should NEVER let that imaginary beam touch anything that won't stop a bullet (that includes any wall, ceiling, or floor) or ANYONE (that includes you own hands, legs, or body) unless you are in a defensive situation and all criteria is present for the use of deadly force.