As I mentioned elsewhere, I have become a big fan of stem type casters for tubular constructed items. Either with the push in style holders, or threaded stem and weld in a nut or drill a hole. And while I'll probably get some static about hatchetwork, I have all but stopped welding on plates to mount casters, and instead just weld the caster flange right to the object. Especially for shop type items. I just about never need to change a caster, if I picked the right kind to start with, and if needed I can always just cut them off and weld on new ones. It doesn't take much to keep them in place, and no fasteners to shake loose over time.

As to gas saving, using just enough gas for the type of weld, minimal post flow, a short, small rigid hose between the bottle and the machine, and learning to plan welds, so you make a contiguous beads, and avoid lots of starts and stops, all contribute so getting the most from a bottle. Early on you will go through a lot more gas but as you get better that will change.