If your planning on a very very thin oil finish it would be okay to leave it and do the whole heel as well, but not for a set neck. The glue won't like binding to an oiled finish. But for a screw on neck if your building up layers of say TruOil, then it may be a better idea to mask off accordingly. Whatever amount of oil is on the heel, it's going to raise the fretboard closer to the strings by that much, if that makes sense.
I usually dry fit the neck and then lightly trace around where it meets the body on all sides in pencil. Also in your case the sides of the neck between top of the body and the binding. Just trace it all around if you know what I mean. I then mask up to the pencil line, carefully following the curves. Once it's masked up, I then rub out the pencil line with an eraser, so it doesn't get immortalised in the finish![]()
Hope that helps! Keep asking away if I have confused you.