I always warm up with 3-4 sets of extensions before doing any heavy lifting with my quads. I think it helps.
Sometimes I'll do both extensions and leg presses before squats to really pre-exhaust the quads. This is so I won't be able to go too heavy on the squats, due to my bad knees. Boy, will your quads scream in agony, if you do this. So give the pre-exhaust thing a try. Its worth it.