You can also think of a heated manifold as a COOLED manifold... really it's a temperature-stabilized manifold. I too live in Los Angeles, and to be honest, I've never detected any difference in carb behavior hot or cold here. But I imagine manufacturers selling cars in Alaska and Florida can tell.
I was really curious about intake charge temperature so I measured it! Info is about 2/3rds down this page:
http://wps.com/AMC/195.6OHV/Intake/index.html
