It's personal preference, mostly. You make the bator, so you can decide. However, if your water trays are on the bottom, you will want the wire over it so the chicks don't fall in and drown, and if yor water and heat source are on one side, you should put up a wire partition to keep a chick from drowning or getting burned.
I did in mine just because its an old chest frezor and the wire made the floor flat strait across. I also added another fan cause its so big and my hatch rate went from 50% up to about 90%. And the temp stays more consistent.