Squatting ass to grass as they say isn't for everyone. Some people lack the bony advantages that are needed. to allow the body to do this. Dean Somerset shows a few videos in this ARTICLE that allows you play around with some ideas and see if your squat cleans up and you produce less butt wink.
First you may wonder why is butt wink bad? Well when the sacrum tucks under, it produces lumbar Flexion. This in itself in my personal opinion isn't bad when you are just doing a bodyweight squat also known as the third world squat. But, flexion under load has been shown to produce ligament strain in the lumbar and SIJoint. Probably not good long term with load after load, rep after rep.
This video Chris Duffin offers some great advice. I myself see more patients/athletes unable to maintain bracing in the bottom in women or men that haven't lifted before and hip mobility in men that have lifted in the past. (bad mobility from junk training)