You should tuck your pelvis during squats, as a small amount of posterior pelvic tilt also known as tucking the pelvis is normal at the bottom of a deep squat.
However you should avoid excessive posterior tilt, also called "butt wink", when doing squats, especially when under load.
The butt wink during squats occurs when your pelvis tucks under, causing your lower back to round, and potentially increase your risk of injury.
During squats, your pelvis should ideally maintain a neutral position, which means that your pelvis is aligned with your lumbar spine.
Excessive butt wink can place extra pressure on your lower back and could lead to injury, especially when lifting of heavy weights.
The butt wink is excessive pelvic tilt which happens at the bottom of the squat, causing your lower back to round.
A rounded spine in a squat can reduce your core stability and power and make it harder for you to lift heavy weights and can increase your risk of injury as well.
If you do experience butt wink during squats you can try widening your squat stance and improve your hip mobility, or even practice exercises which promote core stability.
So while some posterior pelvic tilt or tucking your pelvis during squats is normal during deep squats, you want to avoid the excessive tucking or butt wink to maintain proper form and reduce your risk of injury, especially when you're lifting heavier weights.