I have sort of a combination of quilt and bivy.
I have always had the zipper down the middle. I think it works much better. (But then I have had little experience with side zipper).
If it's warm, I unzip and sort of roll under the sides to get them out of the way but out of the dirt. And there's a large barrier between the dirt to the sides and inside my bag.
It goes about half way down the front.
Theoretically, it's less waterproof, but the zipper is at the top of "a ridge" so the water flows away from the zipper. And I normally have a tarp so I'm just stopping rain splash which there isn't much of at the zipper.
Theoretically there's a cold spot at the zipper, but I wear an insulated vest also, so between the vest and quilt there is no noticable cold spot.