Well, since no one else has answered yet, I thought I'd give it a shot. I think you should be able to do your decreases by using k2tog and p2tog to get gradually narrower ribs as you get toward the top.
For example, you'd start with *k4, p4*. Your first decrease round could be *k2tog twice, p2tog twice*. This would get you to 2 x 2 ribbing. Do that that for a little bit and for your next decrease round do *k2tog, p2tog*. That'll get you to 1 x 1 ribbing. From there, if you still need to decrease, you'd probably have to change to *k3tog, p3tog* to stick with 1 x 1 ribbing or if that looks funny, you could tyring changing to stockinette.
Alternately, your first decrease round could be someting like *k2tog, k1, p1, p2tog* to get 3 x 3 ribbing. Not quite as simple, but it might look better if the decrease rounds are spread out more. Anyway, I think you get the idea.
I've not tried any of this out, but I think my reasoning is sound. Try a swatch and play with it. You might want to use SSK instead of k2tog and see how that changes things. I think you could get a light spiral effect or a pretty symmetical look depending on how you mix things up.
There is no right way to knit; there is no wrong way to knit. So if anybody kindly tells you that what you are doing is "wrong," don't take umbrage; they mean well. Smile submissively, and listen, keeping your disagreement on an entirely mental level. They may be right, in this particular case, and even if not, they may drop off pieces of information which will come in very handy if you file them away carefully in your brain for future reference.