Hey @wilberolive!
Don’t worry, this doesn’t matter for overhead because the thing you’re casting to is guaranteed to be loaded in, therefore it’s practically null impact.
You’re forgetting #5: Get a value and set it into the blackboard using “Set BBkey as Float/Int/String/etc” nodes any time something the behavior tree USES is changed. Don’t store “current walking goal vector” on the AI character; that’s something that only the Behavior Tree needs so keep it on the BB. 
This then makes the rest of the Behavior tree able to call on the variables easier, as it is now stored on the BB, and if you store it on the character then you’d have to constantly be checking the character’s variables with a Service node. Right?