We actually carry the different types of canvas as well as the stretcher bars. As far as I'm aware, the canvas should shrink that much of it's the 100% cotton while the cotton/poly blend is great for keeping its size after printing and coating.
Our Simply Elegant Everyday Matte Canvas is 100% polyester fibre that comes in longer lengths most other canvas and great for stretching.
It really depends on the material the canvas is made from. Most canvas that we carry can be made for doing gallery wraps. Like Epson Exhibition Canvas (the replacement of Epson Premium Canvas), many customers didn't like the way it is since it's thicker and it's not as easy to stretch.
It's basically user preference for these. We've tested the different products and haven't had too much of problems stretching most of these supplied canvas's.
If you're curious on the other options and durability, feel free to contact firstname.lastname@example.org