I, myself, have been guilty of asking people through PMs for their thoughts on information specific to what I am looking for. Sometimes it helps you to figure out the path to provide better information through the public forum.
Thanks for the various comments, people. It's been interesting.
Just to clarify, I'm definitely not talking about the use of the PM function in general, nor am I seeking any kind of policy or ruling - just interested in whether other regulars get these kinds of requests, and if so, how/if they respond. And yes, I use it at times when a conversation is most appropriate between me and one other person, rather than being public.
I quite agree PMs are a case by case basis. And yes, I know I could disable my PM option entirely if I chose; so far I choose not to because there are times when it is appropriate and useful.
The times when I don't think it is appropriate is when I get a request from someone I don't even know, who is most often new to the forum, when they and I have had little (if any) previous interaction, and yet they appear to feel it's quite reasonable (why escapes me) to send a PM asking for a private, individual answer or coaching. In these circumstances, I think the behaviour is completely self-centred, inappropriate and rather selfish.
My rule of thumb remains to say no: & to suggest they pose their question in the public forum, where anyone can read it, and perhaps benefit from the responses posted.