To breathe or where to breathe in Tai Chi that is the question and there is not one definitive answer and yet there is one, let Tai chi teach you.
If you listen, inwardly, when practising your Tai Chi your movement and your breath will communicate and harmonise, or so it should.

When the student has roughly learned the moves, motor skill, muscle memory then they can research where they hold their breath for example in Snake Creeps Down they tend to hold the breath mainly because they perceive that it is a strenuous move.  If they listen and feel what they are doing and they are applying the principles of Tai Chi the breath should be smooth.
So as they do Snake creeps down i ask them to count out loud so they do not hold the breath in and create a restriction in the flow of movement and breath.
In Tai chi all the forward movements should be an out breath and when that concept is understood and felt it should then be applied to push hands.  The research then really begins, for if the ego kicks in the Qi/breath raises and an understanding where the energy is wasted and the bodies internal balance is being compromised.  Touch sparring a more free-flowing two person practise ups the energy used and therefore helps the student understand and feel their breathing under pressure.

Static training, holding postures,standing post, what is the breath doing when you hold a shape/form for any length of time, what fight is going on within, what dialogue is happening.

This is your self-study, self-development  learning to understand the breath by being mindful, conscious of your being.
Bring that all back to the form and listen to the forward out breath/movement, which by the way stimulates the Autonomic Parasympathetic division of the Nervous system the rest and digest, for me it is rest to invest.  

Which is why Tai Chi can be described as moving meditation.

So the up shot is , sense and feel what the Tai chi is teaching you at that moment in time.