You can keep auto-attack on (I do), just walk around constantly to avoid attacking until the creep is low health, then target it. If you hit the creeps at full health, you will push the creep equilibrium towards your opponents tower, meaning you will be more susceptible to ganks (since you're further away from the safety of your tower). Everyone in the game I looked at was auto-attacking, so, if you were to only last hit, you would be dominating the creep score and be rolling in gold.
Yeah, it depends who you're playing as. If you were Slark for example, you would want to be roaming and ganking early. However, as OD, you should concentrate on winning your lane. Astral Imprisonment is such a pain for your opponent, as you steal intelligence from them, drain their mana, and in the time that they're imprisoned, they are missing out on CS. Going toe to toe with someone before you're able to cast your orb without mana issues is asking for trouble. Even then, it can be deadly.
Anyway, I hope this helps.