Rarely. So long as I am increasing the weight i use, I still make good gains. That is to say, I keep more or less the same split, and use several key exercises. That being said, i switch it up within workouts, so that one week i may do machine squats for my 3rd exercise, the next week might be lunges. I always go with squat and leg press first, though, for example.
When I'm dieting, I change it up as often as possible, and keep the workouts short and sweet, to avoid catabolism. I only keep a split for 6 weeks during a diet, so i use two different splits throughout the course of the diet.
