Should you score the concrete slab before staining?  My personal feeling is that an area should be scored if you want to add a touch of class to the area.

A score pattern that enhances the design elements of the space adds to the look.  In general, the larger the "tile" pattern the better.  But, keep in mind that large may not always work in smaller rooms and you really don't want to create a pattern that is too busy.

Hallways, entrances, foyers and bathrooms may only need an accent feature because space is limited.  A laundry room mat be left un-scored or seamless for ease of cleaning.  When thinking about bedrooms, picture the room with the furniture in place and visualize how much of the pattern will show.  Scoring costs extra, so if you are going to cover it all up; maybe leave it seamless.

A border around the edge of a room does two things.  First, it frames the room.  Second, it allows each room pattern to be centered for best visual fit.

Spend some time with your stainer going over what will look best.  Once the concrete is scored, well, it is cast in stone.