Where do I put my thumb??

Hello! I just picked up the guitar one week ago and am working through Fundamentals 1.

I learned the C and G chords but when I play them, I'm confused as to whether my thumb should be placed with its tip against the back of the neck or peeking over the top.

It was suggested in the lesson to press it against the back of the neck to reinforce finger placement, but I notice Lisa in the lessons has it over the top while strumming chords.

Obviously she's advanced and making it work, but I just want to the basics down before I start picking up any bad habits.

A useful video on that very subject here

https://www.youtube.com/watch?v=zQuHqY83mD0

For open chords, I tend to keep my thumb up over the top.

For E-string rooted chords, this tends to mean I'm gripping the neck with the heel (rather than tip) of the thumb.

For A-string rooted chords, I tend to use the thumb to mute the E-string, allowing me to strum all the strings (where I'm not playing a base note first rhythm). The tutorial for (I Can't Get No) Satisfaction (Made Easy) has an example of this.

I was having the same question and problem. Thank you for the link to the YouTube video, it really helped me alot!