Well I did spout off on the topic of engineering and pressure in this post:
https://forum.antiquebottles-glass.com/discussion/comment/852/#Comment_852
Some further thought on this:
Presumably all bottlers packaged their filled bottles in some type of crate for transport. If a bottler really wanted to keep their full bottles on their sides, why wouldn't they just crate them that way? A bit of straw between them would prevent the pile from rattling and breaking each other. It simply does not make sense to me that one would create round bottom bottles to prevent upright storage. Likewise for a merchant selling the wares - he could stack them on their sides if he needed to.
Therefore it seems far more likely to me that either this was a marketing technique (round bottom bottles look very different and were eye catching) or that there is an engineering principle here. That is, the round bottom bottle failed less often than a flat bottom bottle and that was good for business if you were a major soda bottler.
I would love to hear other input on this!