I usually recruit and talk to companions as soon as the options become available to get their side quests. Recruiting agents nets 100 influence each, and each item for a companion's fetch quest is worth 40 influence. Recruiting Ritts, Anais, Tanner, Clemence, Lord Berand, and the Blades of Hessarian, yields 600 influence, and all can be recruited before you even reach a decision on which group to decide upon in the mage/templar war. It is possible to have all 4 knowledge perks by the time you reach Skyhold, but you can't do that and get the Deft Hands perk, which I find more useful because it unlocks doors that have XP bearing codexes behind them, in addition to some really nice loot and schematics.
I don't think you should think about the high dragon before you get to Skyhold, unless you like being roasted alive. I also think that while requisitions are fantastic at 200 influence each, filling them depletes resources and it might be more helpful to craft better arms and armor than what you find so you can do more active side quests with larger rewards later. At higher levels, with resources that have been grown out of, getting new requisitions is a way to turn stuff you're not using in for influence, and you can almost get a perk just walking back and forth between the requisition officer and the table.
I think the XP gained from finding codex entries nets a max of 150 each. You can also get an XP bonus with every combat with Cullen's M__________ Method, right under the Underworld Knowledge perk. I think it's a value judgement as to whether it's worth giving up a different perk if the difference is only 25 XP. My inner rogue thinks losing a few XP is better than not knowing what's behind the locked door, but that's just me. I can always pick up more codexes later.