It’s never a good idea. I scanned my first rejection letter for clues as to why my story wasn’t good enough. I agonized over the phrase “not right for us at this time.” Did that mean I could send it back later? As I accumulated several more letters, I wondered, what did “your story didn’t grab me” mean? Did I need to make my plot more moving? Maybe I needed more action? Romance? I even got a rejection letter that said, “I almost bought this, but I’m going to have to turn it down.” Almost bought? Did that mean I should make it 20% better somehow?
Over the years, I’ve come to a conclusion. Even if your name is at the head of the letter instead of Dear Contributor, these are still stock phrases. They don’t mean anything. Unless an editor or agent specifically says what didn’t work for them (i.e. I found your main character too harsh, I was looking for a plot that revolved around a single main character) then they’re not saying anything at all. Why don’t editors or agents give more feedback? That’s easy. They don’t have time. And the fact of the matter is, there may be nothing wrong with your story. That’s right. They rejected it, and there’s nothing wrong with it. You may have written a fantastic story about robots, but they just bought a story about robots, and they can’t have two robot stories in one month. You wrote a perfect book that wasn’t an exact fit for the agent you sent it to. Or maybe you didn’t have enough writing credentials. Maybe, maybe, maybe. The important thing to remember is, don’t go tearing your story to pieces, looking to make it fit the advice you think you’ve gleaned from a rejection letter. Let me say it one more time: your story got rejected, and there may be nothing wrong with it.