Thanks for the information so far, everyone. If there's any more, please continue on.
I won't be making the leather seat myself, of course. I know nothing about it and will leave that up to someone else later, when that time comes (I'm months away from that stage), to supply a seat. I'm just trying to determine where to place the wooden stretchers that the leather seat will attach to so that it won't immediately "bottom out" so-to-speak. Over time, I guess it will and will either need to be replaced or repaired.
I think the consensus is that any leather is going to stretch to some degree depending on thickness and type of leather used. Maybe even some lamination process with other materials to help slow that down. Very interesting. But, like I said, I won't be doing it myself, but at least it's good to know for when I'm looking for a leather-person to do it.