I like to see some kind of upward trajectory in my rejections (either takes longer to come through or comes with personalised comments) to keep submitting to the same place because I figure the editors are only human, and if they've flat-out rejected me ten times already they're bound to have preconceptions of my eleventh story when they see my name on it. But I don't submit often enough to really think I'm clogging up anyone's slush pile too severely so I usually start at the top (Clarkesworld etc) and work my way down, because you never know.