Well, if the map is extremely wide or tall but very small in the other direction, you could keep the whole thing as a single map since its actually memory size wouldn't be that big. I've personally submitted a map for the NES that was 27200 pixels wide, but only 240 pixels tall.
However, if there are things on this map that could serve as breaks like the occasional bosses or barriers or whatever, it might be a good thing to split it after those points. I'd still put all the segments side-by-side in the same image though to show how they're all part of the same area.
*EDIT*
Will answered better then I did while I was writing my post. For Gradius (NES), the maps are always a whole number of screens in size. The automated scrolling would stop while fighting the boss and resume afterwards, and I split the stages right after the point where the scrolling would stop.