I think the general question raised is not so much the proper use of qualifiers (IMHO, IIRC, etc) but rather communicating clearly, concisely, and most importantly, using the proper tone.
What is the proper tone? Persuasive when trying to convince, open when asking questions, tactful when giving feedback, reasoned when disagreeing, and throughout: civil, polite, and respectful.
And as Art mentioned, modern tools like email and forums drain the nuance out of communication, particularly when so much is quickly dashed off and submitted.
So, I agree with pretty much all the opinions expressed so far: should qualifier be used in posts? Yes, if they help you communicate more clearly and with the proper tone. Are they necessary? Not at all, if your writing is skilled or you spend extra time editing/revising your posts to make your point.
By the way, the absolute best reference I own for writing is Fowler's Modern English Usage (2nd ed). A gem of a book.
with best wishes to all,
Tom