My tires are 700x45 (47-622). Bike shops here all stock either a 700x35-42 or 700x37-43. Nothing larger. All say these will be fine. At least a couple of times when I'd encountered this in Europe the shops were very much against using undersize tubes and would recommend going to another shop if they didn't have the right size. They said that stretching an undersize tube causes much faster natural air leakage (instead of having to add air once every three months you'd need to add it about once per week) and seemingly they flatted easier.

Any truth to this?