It is different depending on if you are by your self or not and how you unload. If you back in, climb aboard, start the motor, then push off, you need it deep enough to pick up water while running. If you unload with a rope, leave it up until you are in the boat ready to get underway.
When power loading trim up enough to keep the motor from hitting the ramp as you move onto the trailer. This will also raise the bow and facilitate the loading process.
Repetition makes everything easier.