I'd second this: the case tends to be a bigger constraint than the board (which you can modify at the design stage). Even when you are fabricating the case, the manufacturing constraints are usually more difficult.
Although sometimes you may want to get the pcb working before worrying about the particulars of the case if there are a lot of unknowns in the design.
Although sometimes you may want to get the pcb working before worrying about the particulars of the case if there are a lot of unknowns in the design.