I have used very narrow tubes made for 700x23mm tires with 700x32mm tires and never had a problem, I have also used tubes made for 28mm tires with my 38mm tires and it was always ok. Tubes stretch. Actually I prefer to use smaller tubes because I carry 3 spares and smaller tubes take up less space. In my experience smaller tubes also make tire removal/install a little easier with some types of tires. I think that using tubes which are too big would be a more serious problem then using the smaller ones.