I would expect that most people close multiple tabs with [cmd|ctrl]-w, precisely because of the back and forth mousing otherwise required. One thing I've worried about while using Chrome has been accidentally closing more than one tab at once, since, being a klutz, I don't have wonderful control over my index finger, and triple-clicking to select a paragraph is so common that I do it accidentally sometimes when intending to single click on a button or something. So far, I don't think it's actually happened very often, though.