Do you mean it tucks in, or tucks out?
When I squat at the limit of the weight I can handle, I sometimes stick my butt out at the bottom, shifting the focus somewhat onto my lower back (sticking out the butt also means lowering the upper torso a little, rocking onto the front part of the foot, and changing the angle at the waist). But I'm only talking about a few centimeters here, I think.
This is something that I battle and work to fix & avoid, and is something people who know how to squat will notice. But it's no reason to hold off upping your weight, I think. It's just something to work on as you get stronger.