I am using an office chair as well. The arms are adjustable up and down and they go down far enough that they don't get in my way when playing. Being an office chair, it has several adjustments thereby giving me good back support. Also find that it is comfortable which makes me forget to get up once in a while to stretch!
As for your other question, I don't normally play standing up. But I just tried my telecaster standing up and didn't notice any dipping as I played. Perhaps it is only because I played a couple of minutes standing up?