The casino set-up really controls this more than anything. When they have a 'room' the intent is to keep the table play semi-private. Few (if any) rooms will let you just walk in and stand around. These aren't blackjack
or craps tables in a high-traffic aisle.
Wherever even an imaginary 'rail' is established is the 'no pass' line. If you do happen to find a poker area wide open, I wouldn't stand within 2 feet of a live table and never, ever crowd a player's seat.