Definitely a backer for me!
In addition to strength, it can help with warping over time. I usually leave the backer 1/4 inch from the edge on larger pieces or 1/8" on smaller ones.
One other note. For me, I don't skimp on the backer for nice intarsia. I figure that if I'm using exotic and often expensive woods for color and grain and putting all the time in to create a piece of art, I won't use cheap luan for the back. I usually use nice baltic birch. I also sand and give it a light coat of satin lacquer to the back of the piece.
Ray