Personally, I realized that chests are better be opened whenever you stumble upon them. For example, if you wait until you're level 20 and go back to Cyseal to open a few chests you chose to skip when you started the game, the loot you will find will mostly be level 5-8 or so. It doesn't scale to 20... there's a limit! Therefore, just open everything you find and don't think too much about it! The best gear you'll get often comes from high level vendors, and those do scale with your level, up until 20! Chests stuff is most of the time meant as another form of cash.
