The importance of a "sealed" room on sound quality?

[Uniquely]

When I chose which room to set my HT up in, I chose my living room over my family room. The main factor in my choice was the fact that when I close all of the doors to that room it was completely shut off from the rest of the house while my family room is open to the kitchen and the hallway that leads to the rest of the house. The downside was that the living room is a MUCH more awkward shape than the family room and speaker placement has always been a huge challenge for me. I'd just always heard that having a "sealed" room was much better for sound quality because it allows the sound to "pressurize" the room.

Is this true? If so... any of you sound guys care to explain the dynamics of that? If you had to choose between a perfectly shaped but completely open room vs an oddly shaped totally sealable room... which would you choose?