This is a really interesting topic Pete and, I must confess, as someone who has only recently come back to "serious" photography after a previous heart-hearted attempt 18 years ago, it is not something I have given a lot of thought to.
However, I have a cyber-acquaintance who is a semi-professional and he touched on this subject in a recent blog of his. It's quite a long piece so I won't transpose it here but
here is a link if you'd like to go read it.
I think the interesting point he makes is in the penultimate paragraph
with rights come responsibility. Just because you can do something doesn’t necessarily mean you should and I think that sums it up in a nutshell. Judge each situation on it's own merits and decide yourself what is appropriate and what isn't.