Most of the time we need to use all four fingers equally and therefore the thumb needs to support the 3rd and 4th fingers as much as the lower fingers.
It's generally best to place the thumb opposite the 2nd finger, the 1st finger or somewhere in between the two to help develop good finger-fall action in the 3rd and 4th fingers. There can be variation depending on the length of fingers and thumb.
Our hand position and therefore our thumb placement need to change slightly as we play to suit the task.
We must take care not to press against the violin neck, this will cause tightness in finger action and shifting. You can release tightness in the thumb in various ways:
Tapping the side of the violin neck
Polishing the neck
Using a lighter finger action (half harmonic touch)
Left-Hand Fundamentals - Bring the Violin to You
Finger Action - Introduction for The thumb