The arrowhead should be integral with the line, so that the end of the line and the end of the arrowhead is the same location
While I agree with this fundamentally, in practice, where I can see it being a problem with where the end of the line (square) exceeds the width of the tip of a pointy arrow. Now it's not pointy, but has a few little corners sticking out.

I suppose what I'm saying is that it could be looked at either way, and while the sizing thing is sort of odd, I'd rather have to think about that than deal with arrow heads that could only accomodate a certain line width before they got "corrupted".