It depends a lot on the context. I wrote an open source VPN client and although I run a mailing list to which anyone can post, without having to subscribe, and I clearly invite people to do so if they have problems, I still occasionally do a web search and find that people have been asking questions on random web forums instead of asking me (and my other users/developers, who can also often help) about the software I wrote.
I'll often go to the trouble of signing up for these web forums just so I can post an answer to the question, which is often months old and unanswered by the time I see it — although it would have got an immediate response if asked in the right place.
And when I do, I'll try to give a simple answer if the answer is obvious and doesn't involve a debugging session with the user. But often, a person who is confused enough to post their question in the wrong place is also confused enough that they don't include enough (or indeed any) of the pertinent debugging information in their post either.
My experience is that with these random web forums, I often won't get decent notification of a reply. They often send from bogus addresses which fail sender verification, or just don't send notifications at all. So the a round-trip of me asking a question, the OP answering it, and then me eventually thinking a week or so later that perhaps I should check this web site again if I can actually remember where it was, can take a very long time.
So I will definitely always tell people asking about my software on random web forums (this one included, although it's one of the saner ones) to use the proper channels for support, if they want me to help them.