The last stage of shedding where the skin is being removed from the body should happen pretty quickly. If the skin is still stuck to your snake I would assist with the shed before feeding. Quite often snakes shed, urinate and poo all right around the same time. So if the shed is stuck right over their vent, they may not go and may just hold it in. As you can imagine this isn't a good thing if they really have to go.
They can also go and leave you with a snake with poo all over it because the shed got in the way. Both scenarios are ones that I'm sure you'd want to avoid.