Stretching a smaller tube to fit a larger tire with air pressure is not a whole lot different than using a larger tube that starts out thinner. It is one way to save a little weight without having to find thinner tubes in the larger size. Air loss (even without a puncture) will be a little faster. I wouldn't hesitate to do it. And as mentioned above, it does make mounting tires easier as the smaller tube is easier to stuff inside the tire.