Buck Turgidson wrote:

When putting up CM in a simple 4 sided room, what's the best way to deal
with the last piece - i.e. does one use a coped joint on each end?

I was considering putting up the first piece with one end coped, and fitted
over a small piece of scrap on the adjacent wall, the scrap being a proxy
for the last piece. Then when I get to the last piece, slip out the scrap
and slide the real piece in there. Am a little concerned about it being too
tight, though.

Depends on the size of the room...if one piece will do both wall
lengths, then that's the way I tend to do it. If need two or more
pieces, I make the joining miter to length. Your "split it in" way is
possible, too. Just don't nail the other piece in all the way to the
end and you'll have plenty of slack unless it is a very wide mould...

And, if it is more than a couple inch wide, you are using support
blocking, right?