We all know the golden rule in Minecraft, "don't dig straight down". However, this rule does not need to be obeyed if let's say, you have full enchanted diamond armour with fire prot on at least one of the pieces (since the main reason behind the rule is the possibility of falling into lava). When you're far enough in your gameplay, why the heck not, dig straight down. Just keep a water bucket and a couple of fire res potions on you - along with the essentials of course (food - except on peaceful, armour, weapon, pickaxe) - and you'll be fine. 

Same with digging straight up. The main reason behind this rules is that sand or gravel fall on your head, or you wind up in an ocean and drown, but with a good shovel and an aqua affinity enchant on your helmet, you can avoid this rule.

So go ahead, dig straight down. And go ahead, dig straight up.