Where I stand, if I copy a chunk of code from someone, then I'll mark it to say where I've copied it from. Else if I see an idea/way of approaching a problem, I might elaborate on it, but I would always be constructing the code myself from scratch, and hence would not deem it necessary to mark it. That said, for each of these there are fuzzy boundaries - and so, no, I won't be drawing any lines.