What is the functional difference between designating a room with a bunch of beds as a "dormitory", vs just having a room full of roomless beds?
I left a few beds laying around, built but unroomed, and seen dwarves sleeping in them, and they get the same thought they get from sleeping in a dorm. I suppose building dorms will make them more likely to sleep in them, but i do not know for sure as i don't build dormitory's.
In regard to Bedrooms vs dorms, i always try and make sure my dwarves all have good-quality individual bedrooms(though i sometimes forget), as there is a great happiness boost from doing so. Given the amount of crazy stuff that happens to your dwarfs, and that if they get stressed enough even more crazy stuff happens, i belive this moderate boost of happiness is than the worth the effort. The effort required is rather small in any case, mining levels up fast and your carpenters/masons/metalworkers are generally pumping out tons of furniture by the time they get to near-legendary anyway. Designating the bedrooms(and worse adding furniture)takes some time, but not a truely great amount, and while i have not tried this myself i believe you could automate this with Macros(something i aim to try with my next fortress.)
So in conclusion bedrooms are the way to go if you want to play it safe, of course, often one does not want to play it safe