Detailed above. The only reasons to deviate from the order is if the enemy or allied support is Taric or Leona. In this case you want to put 2 points into arcane shift, as this coupled with mystic shot's passive means that your Arcane Shift's cooldown is ~13 seconds - the same as Taric's Dazzle and Leona's Zenith Blade - providing you use your Mystic Shot as soon as it comes off of cooldown.