When pole standing, should my weight be balanced more on my heel (where I feel it works my thigh muscles), or the ball of my foot (where it seems to work more my lower leg)?
I'm not an expert, but there's a discussion on weight distribution here that suggests that the weight should be balanced uniformly.
Keep the center of the feet (Yongquan points) lifted lightly and place your body weight exactly onto the center of your feet.
Actually, you open the Yongquan point by grasping a very little the ground with the toes. It will lift up a tiny bit the arch of the foot. Do it first with your intention only and then add a very light physical gesture. Nevertheless, the weight can be distributed quite equally all over the feet. Put down first the heels, then the exterior edge of the feet, then the toes -from little to big-, grasp lightly the ground with the toes (without tension), and find the center of your feet, and settle your weight there.
Your knees should be bent, and your weight centered over the arch of your foot, which is near the center. This places weight on both the ball and heel of the foot.
One goal of zhan zhuang is to eliminate unnecessary muscular tension used to hold your body up. Although you may feel strain in your lower leg for some weeks or months, this is not a long term feature. Over time you should feel less strain and your body should feel like it holds itself up through structure.
People who do not bend their knees tend to stand with their weight on their heels, which tends to cause stiffness in the lower back. I have never heard/seen advice to center your weight on the heels; this is often enumerated explicitly as a common mistake.
Your weight shifts forward from the basic position to over the ball of your foot. You can allow your heels to rise by a sheet of paper's thickness to ensure this weight shift. Personally, I think this should not be done without instruction; the basic version which can easily occupy you for years.
"Opening" the yongquan point (or any other point) is not a strictly mechanical process. You learn what this means by practicing consistently and feeling how your body develops over months. It's not sufficient to put your body in the right position.