The best position for a Beginner is having your thumb behind the neck to strengthen it so you can play barre chords later on. Justin even mentions this in his early lessons.
Watch both these videos and what where their thumbs are. on the open chords the thumb is above the neck by Zakk can clearly play barre chord because of the strength in his thumb.
In Both videos both Zakk and the other guy have straight wrists
https://community.justinguitar.com/t/barre-chords-with-hammer-ons-and-pull-offs/79193