There’s not really a best position for your arms unless you’re talking about a specific lift. There are a lot of muscles in the arms, and the reason for that is to provide efficient movement at a huge variety of joint angles. And if you don’t train using a wide variety of joint angles, you’ll never have a balanced physique (looks OR strength).
So for example, you should train triceps at multiple shoulder, elbow, and wrist angles. Over the head, with wrists pronated: works the long head. At the midline of the body with the wrists pronated (skull crushers): works the long and lateral heads. At the midline or lower, with the wrist supinated (reverse grip crushers or pressdowns): works the medial head. Pronated grip with the elbows down (pressdowns) works the lateral head.
PS: The first sentence of the OP is not necessarily true. For Powerlifting-style squats, yes. But look at the knee, ankle, and hip geometry of the leg press (generally thought to be safer than squats by personal trainers everywhere).[/quote]
I understand the position of the hand and upper arm changes what muscles are worked more but i see in some threads that people with shoulder problems like benching neutral grip db cause it doesn’t cause them any pain or less then anything else. What you said about the leg press. You can have your feet wide narrow w.e you want and still have you knees in line with your toes. Even on non-powerlifting squats knees should never buckle. Meaning knees should follow a certain path right? After all technique is first then adding weight.