Lucy,
depends on how and where you're going to use them. I just got back from California where I shot the American Rottweiler Club's National Specialty. The stage was 4 feet above the floor and the dogs were posed on the stage while I was stationed on the floor. To get proper lighting height for people and dogs on the stage, I had my light stands extended to full height -- 10 feet.

In the studio, I seldom place them higher than 7 feet. Again, it all comes down to how you intend to shoot and where. Tall, sturdy stands are nice to have because when you need the height you have it and when you don't you can place the lights at whatever height you need.

Jim