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).