Good question...

On my own airline I always wear slacks, button up shirt, jacket and dress shoes.

I do this on any other airline too, with the exception of any US based airlines as it doesn't seem to matter? I stayed dressed up the other day on a DL buddy pass as I knew I would prob be upgraded. But then I used a ZED on them the next day in jeans and got upgraded anyways! I never bother on AA or WN. I heard DL changed their policy to say that if it doesn't get you kicked off as a rev pax, it won't preclude you from a seat or an upgrade.

How much does it matter any more?