When the house is spacious, I play with walls up. In TS3, I often do this since we can tilt the camera such that you see your sims from above. I often do like 75-85 angles with respect to the ground level. Or I zoom to the sims and see what they're doing up close and personal. Walls up is especially useful when I go on adventures. Easier for me to spot hidden places. I love world adventures from among all the other expansions.
But when I'm just skilling or doing mostly routine things that I speed up, it's either walls down or walls up.