As long as I have "control" over the space physically in front of my seat (for either my bag or my legs), I'm fine. If the other pax needs to use the space in the middle cubby that spans across both seats because there's no other space, that's usually fine, but it doesn't mean s/he can use that *entire* space (e.g., the space in front of me is mine...). If the bag won't fit without impeding my legroom, then it needs to go up.