I had a 24" (520) tube that had a crease in the side leading to it getting a flat. I don't know if it was due to long shelf-life, or a defect in manufacture.

But, as others have mentioned, I don't loose sleep over the idea of tube defects.

When I place a mail order for bike parts, I often add tubes to the order. I just snagged a couple of tubes from Ribble for $1.26 each, although they are a bit odd, and not fully flat when packed. I haven't tried one out yet.