Some placement is acceptable, and others are terribly difficult to manage.
Placement should be intuitive for each prefab, and not necessarily the same for every prefab. If I am placing a door, I don't want to be 5 blocks above the door (on the next deck or too high to be able to see where the bottom of the door actually is) in order to place it. I want to be in front of the door, with it facing me as I pace it at the center of the floor it will be sitting on. If I am placing an engine/thruster/weapon, I want the prefab to be placed facing me when I am attaching it to a 'wall' piece, another engine piece, or another weapon.
Being able to adjust the prefab after placement will help greatly as well. I would think an 'unlocked' prefab would move freely without having to hold a mouse button, and maintain its original orientation to the entity when it started (but could be adjusted by hitting a rotate button).