It's my understanding that the hips should stay level throughout, or at least you should strive toward that. I don't think it is a good idea to allow the hips to go out of level in order to stretch for a longer stance. I think in general you will find it more comfortable, more powerful, and easier to transition to the next move if you keep them level pretty much all the time. I will ask Yang Jun to speak about this, but it will probably be a couple of weeks before I can give you his reply.
On a somewhat related topic, a lot of people try to have a very long stance in order to allow their form to go lower. Beware of this. It's important to be able to pick up the feet nimbly. If you cheat and go long, it might be at the expense of nimbleness in picking up the feet.
One point I have noticed recently, a corollary to the empty step weighting question, is that many people have problems with 'low form' (snake creeps down) because they have too much weight on the back foot. If you leave plenty of weight on the front and don't withdraw so far, it will work better and you can go low comfortably. Look carefully at the drawing on the top of each page of the bulletin board. The center of Yang Chengfu's torso is well in front of his back foot (and even in the drawing you can see that his hips are level). I have made it a rule lately for the low form to stick with the principles, trying for correctness, not lowness, and just go as low as is comfortable. On the first round this is seldom very low for me. On the second round, lower. On the third round I am relaxed and extended and it becomes quite low.