Every once in a while I submit something to an editor who wants exclusive submissions. I usually say in my cover letter that it's an exclusive submission for 3 months. I've read that after the 3 months expire I should send another letter, notifying the editor that my submission is no longer exclusive.You're probably right.
I lean toward thinking this would be just another envelope to be opened by some underpaid assistant. Would any editor read my three-months-are-up letter and think, "How I regret missing this deadline! I must dig through my slush pile to find the unsolicited manuscript of this totally unpublished author, before someone else snaps it up!"?
But suppose I don't send a letter. Six months later the editor picks up my submission. She looks at the cover letter and realizes it's no longer an exclusive submission. Have I committed some dreadful faux pas?Not in that case. At the same time, there's no harm in sending such a letter on the off chance that the editor is reading your manuscript just now, or has read it and wants to ask for a rewrite (and is formulating her thoughts first).
And if she is in fact in the process of showing it to her colleagues in preparation for acquisition, she'll be damn glad you reminded her of the three-month mark.
Bottom line, yes, sometimes being courteous makes a difference, and sometimes it doesn't. Sometimes being discourteous makes a difference, and sometimes it doesn't.
But you never know which one it'll be until it happens. Which "makes a difference" situation do you want to be in?