I'm one of those who take the leader out of the canister (135), cut the leader and trim (as explained) the head, and insert the head for around 3 cm inside the reel. I put everything in the dark bag, and work from there. If there is any problem, you can always reload all the film inside the canister (leaving outside just a few cm of head) and open the dark bag and begin the process again.
I don't like taking all the film out of the cartridge first, as I feel this increases film contact with dust and with fingers and risks of microscratches, which I want to avoid (I scan at 4000 ppi and everything matters). Also my changing bag is pretty small and the film would be all around my hands. When I loaded the first reels I had problems and the operation took time and my hands begun sweating in the changing bag.
Now it's all very fast. I do clean my hands with soap anyway not to live evident finger marks on the back of the film.