I run all my circuits from panel out, so if I'm interrupted I can go back and know that if the device has a wire it has a feed.
For bathroom wall lights, I mount a 2x4 behind where the light will be, horizontally, with the top at about the center of the light. Then, I take the switched circuit from the switch to the light and stub it out at the top of the 2x4. When I finish, I cut a pancake box in. It allows a couple of inches of movement if the vanity isn't exactly where I expected and when it's a long way off, I just have one wire to move and a small hole to patch.
