I find that I usually use a 2:1 or 3:1 ratio on most of my images. This being said, I believe that the crop should enhance your composition instead of just fitting into a "standard" size. I often start with 2:1 or 3:1 and then play around with the crop box, fine tuning until I get a pleasing crop.
I like a 3:8 ratio in panoramas, but that's a personal preference (I like to have some height to a pano--I've printed a really wide but not-so-tall pano before...I think it was 14" tall and about eight feel wide, and I didn't like that it was impossible to take in the details without walking up to in in sections. Like I said: a personal preference.
I agree with the others. For me the subject will dictate the panoramic format ratio. David Kennedy brings up a very good point: long pano's with a short height have always looked off to me (for the most part... I have a few that I like). I prefer a pano that has some height to it, as well as length. I've been doing more stitched panos with my camera oriented vertically (rather than horizontally). It packs more resolution into the image, because you have to take more images along the horizontal axis, and typically produces a slightly taller pano. Most of my recent work have been done this way, with a total of 6-8 images providing for the overlap.
I have seen a few long panos with a shorter height that have looked pretty cool. But in order to be appreciated, they need to be printed LARGE. Large panos = custom mounting = phewwwww... there goes the mortgage.