roakey,
I think your pretty much on the money. We use more than one gas supplier. Some charge rent on the bottles and some don't. If I am not paying a monthly fee I can keep more bottles on hand longer and drain them down more. If I am paying rent and it's too long between batches you reach a point where it's cheaper to give back some of the gas. As you know, PP mixing doesn't yield the best economy but I am about the only one I sell trimix to so volumes are low. Having more He bottles obviously lets you drain the first one lower but what about the others? In order to take advantage of having a bunch of bottles you need to keep cycling them through. You still end up with a bunch of money sitting around. If you buy He for, lets say, one trip with two sets of doubles, does it make sense to buy 4 or 5 bottles and store it all and maybe pay rent until the next trip vs buying 2 or 3 bottles and giving some back. It's a balancing act and depends on how long you can let that money sit.
One other limiting factor I found is in my mix controler. I use the Global set-up and it has a flow restrictor in-line. Once the tank gets down below 500 pounds or so, even if the tank I'm filling is empty, the flow is so slow that it drives me nuts. At this point I usually call it empty. Trimix is a PITA but I am hopelessly addicted.
Mike