Here are the greatest lies in golf. Keep your head down, keep a stiff left arm, I'm from the government and I'm here to help you!
Starting with the first one, keep your head down. I'd like to meet the idiot that started that phrase, "keep your head down". I can show you pictures of Butch Harmon giving a lesson to Tiger Woods with a golf club grip up under Tiger's chin which Butch was using to get Tiger to stand tall and keep his chin up. Tiger Woods moves his head back on the back swing and forward on the downswing. Most touring pros have their heads move downward through impact while they are stretching out and low before rising to the finish of their golf swing. Yes Jack Nicklaus preaches keeping your head perfectly still, BUT Jack positions his head well behind the golf ball at address and before he starts the golf club back he turns his head to the right to make room for a healthy shoulder turn.
Trying to keep your head perfectly still will most likely put you in what is called a reverse pivot at the top of your backswing. The by product of that is you will need to pivot backward or away from the target in order to square up the golf club at impact costing you club head speed and yardage.
The second great lie is a stiff left arm, (right arm for lefties). I played a lot of golf with Calvin Peete and I can tell you that he had is arm bent all the time, yes I mean all the time. He ad a accident that caused his arm to be bent as did Ed Furgol who won the US. Open many years ago. Look at Hank Khuene on tour and how much he bends his left arm and he is one of the longest on tour. A little bend or softness in the left arm is preferable.
Bottom line is you should not pay attention to generalities and rules of thumb when it comes to golf. Heck, Lee Trevino used to move his feet at address before drawing the golf club back which would be considered a very nasty habit. He certainly did very well and was one of the best and most consistent ball strikers on tour.