My $.02: Match as you want to match but realize this - The darker the color, the 'smaller' the room will seem. A dark ceiling will accent the low ceiling. A light ceiling will take the eye 'away' from it. So, if you can, go with the lightest possible color for walls AND ceilings when painting small rooms.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!
JayJ is right in what he says. Let me add my two cents worth tho give you a better understanding. First the reason a dark ceiling and dark walls make the room seem smaller is a process called light transference. Light itself is measures by "lumens". Believe it or not, light travels and rebounds much like those pink rubber balls I played with as a kid. Rule of thumb is the lighter the walls and ceiling, the more light the room will seem to have. The darker the room is the less reflected light you will see and therefore the room will look smaller. It all depends on the effect you wish to acheive. I had this problem on one of my rehabs. The lady wanted dark colors for a soft look. We solved her problem by installing wall mirrors on one wall. This can be done very tastefully and the light reflecting off the mirrors made the room seem larger than it really was. But in any case, it's all academic. It's a matter of the finished look one wishes to acheive. Play with ideas and try them out on paper first.
Good Luck, Len
Personally, I would match the slant to the ceiling. The more light reflective surface one utilizes, the brighter the room will be.
Just a note: I had one customer whose son painted his entire bedroom flat black and then they both wondered why the kid's eyes got fatigued trying to read. LOL