I have learnt 2 forms of T'ai Chi - one based mostly on Ch'eng Man Ch'ing and the other is traditional Sun style.
In Sun style there is no specific breathing in the postures except for the open/close movement, where we breathe in as the hands open and breathe out as they come back towards each other.
I asked John Kells, my main teacher for many years, about breathing in the form. Good idea was his answer.
Natural breathing was the advice given by both my teachers. By contrast others practise reverse breathing and coordinating the breath with the form. I have an open mind on this but find myself in sympathy with the following from " Imagination Becomes Reality " by Stuart Alve Olson, Dragon Door Publishing 1992 on the teachings of Master T.T. Liang ( one of John Kells' teachers).
The advice given is to forget the Breath and instead pay attention to the Mind - Intent, which is focused on the Spirit of Vitality. If we focus on the Breath we will have stagnation. The Breath and Ch'i will develop naturally.