I like to paint on unstretched canvas. I have done that for years. The painting can be stretched later or not stretched at all. Easy to store paintings that way too! Easy to ship!
But, alas the canvas I have purchased lately has been, well, crap. When I go to paint on it, the canvas buckles and warps. I tried taping it to my table and that helps, Sort of, but certainly not ideal. I am used to working on many paintings at a time and if my work table has painting taped to it, then I have to clean house, Oh! The horrors!