..When Private messages are or may be used without breaking forum etiquette ? any ideas?

Are you asking if PM's can be be posted on the open forum ??

Generally NO unless the sender gives you specific permission to do so.

PS- if you get any unwanted PMs you can place the sender on Ignore and/or send a copy on to the Administrators here.

Forum support team link- see Administrators listing
http://neurotalk.psychcentral.com/showgroups.php

No I did not mean posting PM on forum. What I meant is simply sending a PM. I was wondering how common private messages exchanged between forum members?
On occasions I was hesitant to use PM's for fear of breaking "unwritten" etiquette

I think normal rules of social etiquette apply: Be polite.

Beyond that, it's probably considerate to have a reason for sending a PM instead of posting. I've received only a few PMs and I've sent even fewer, mostly because the occasions where I have something that I don't want to post are rare.

I suppose one might have a subject that they consider private or embarrassing and may have a sense that directing it to a particular person may be less threatening or just safer. That would be a good reason for a PM.

I'm sure there are other good reasons to PM.