I'm curious as to why you want to do this? If you're doing any sort of real calculations with the numbers (or to calculate them) then you should have some sort of idea of the number of significant figures you're dealing with from the original source.

Otherwise, I'm guessing this is just for aesthetics of presentation? I guess I'm just too steeped in scientific ambivalence when it comes to the beauty of numbers. I'd just pick either the number of sig figs you want to use, or the number of decimal places, and use sprintf.

This is not criticism -- I am curious as to what you want to do with your numbers.

Matt