When doing back squats I feel a little unstable coming up. The weight shifts from my heels to the side of my feet and I have to try very hard to concentrate the weight around mid foot, and not let it move forward.
I read in other posts what the weight should always be on the heels. But from reading the starting strength, they always describe the weight as being "over mid-foot". So should I feel all the pressure on my heels, all the time? Or over the mid foot?
These are independent variables. The barbell should remain positioned in space in the plane defined by the line between the mid-point of each foot being extended vertically upwards.
Without changing that well-agreed fact, there is a cue to keep the weight in one's heels while squatting, in order to prevent shifting onto the balls of one's feet. Notice that you can keep the bar positioned spatially over your mid-foot while shifting your footing such that more weight is on your heels, toes, outsides of the feet, and so on. The cue of "squat in your heels" is meant to encourage good form, including keeping the barbell over the mid-foot plane.
You should endeavor to push the ground through your heels while simultaneously keeping the bar tracking over the middle of your foot.
