I stand both for casting and reloading.
While I've never had a nasty spill or full blown tinsel fairy visit, there have been enough concerning incidents that I want to keep the option of moving away from the pot quickly.
Maybe I just never have gotten the height of my reloading set up right, but I find the best leverage standing.
My feet and back seem to hold up for the couple or three hours I can go before my attention wanders or I get tuckered out, but I guess the door mat in front of the pot on the outdoor deck where I cast and the interlocking closed foam cushioned tiles over the concrete basement floor where I reload work are helping there.