If it fits in the Rotatrim, that's where it goes. If it's coming off my Epson 9800, I simply use cheap scissors purchased on sale at the box home improvement store. For any cutting where the edge isn't critical, I'll still use those scissors.
If I'm cutting the length of the piece, I simply put cardboard on the floor and use my 48-inch straight-edge and use a single-edge razor blade. When it's longer than 48-inches, I simply cut about the length of the straight-edge, then move it the next length to cut the next section. It's not fast, but it seems to work.
What we used to do to split the web at the newspaper years ago when the paper was too wide was to put a box cutter or single-edge blade in the path and let the paper go by and split itself length-wise. But that was years ago and this was newsprint, not canvas.