The point about either cutting in or building on applies in general - basically don't zigzag across that earlier edge. Think of it as though, in the case of leaves/petals, like you have a physical flat material in front of you. Whatever you do is going to either attach to the existing structure, or you're going to cut back into it - but the previous phase of construction is still going to have considerable influence on the result. It's not a suggestion you can choose to follow where it suits you - it's a solid part of the construction that you're working with.