The only low level (ie: not ilvl 70) gear you need to keep in your stash is the lowest level socketed helm and wep you can find. It doesnt matter what the stats are, just need sockets. Gems have no level restrictions. So a flawless royal in helm is 41% bonus exp, in wep its +280 dmg. +280 wep dmg and 25 paragon points in all resist will allow you to faceroll everything up until level 50 or so (assuming you upgrade your gear with drops along the way). Makes life a lot easier. Also really good to keep is ilvl 70 lvl reduction weps. They roll up to -30 lvl reduction and you would not need to use a different wep until you reach 70.
This is important. We don't want to make it sound as if you can just wear level 5 items through level 50 just with +280 gems. As long as you're upgrading all your equipment with drops and crafting as normal, the other main benefit is being able to raise the difficulty level early on. I don't remember the last time I even started a character on Normal.