If you butt them together you leave no room for expansion. The backerboard will not expand, but your walls will. If everything is butted tight and your wall expands into the room guess what happens. (And its right there on the sticker so dont tell me you couldnt find it.) Start your screws in the center of the board and work out. This eliminates undue stresses on the boards. Notice I said typed properly installed? Laying them down on the floor and shooting drywall screws through them does not constitute proper installation. Choose your weapon. I prefer Hardiebacker or Fiberboard. Backerboard placed into thinset and screwed down. Your floor is probably too thick (should be) for the backer screw to actually penetrate into the floor joist. If not, or just to be safe, do not place screws into the area above the floor joists. Working from the center out eliminates that. It would probably never, ever be a problem but if youre anything like me your installation would be the millionth one for that one in a million occurrence.
If you screw all the way around the outside and it is not perfectly flat you are going to have to release that pressure somewhere and it. Backerboard screw wont happen until you have all that pretty tile on top of it.