Best practice for dynamics position on SATB hymn with chords

[Yann]

Good evening everybody,

My usual layout when typesetting hymns is a ChoirStaff, with SA-TB
voices, main lyrics printed between staves, and chord names printed
above the ChoirStaff. In some hymns that have some "echo" voice, if
lyrics differ for the echo, they are printed below the ChoirStaff.

I was wondering, what is the best practice for positioning the
dynamics ? Above or below the staves ? Should them be printed only
once for all the ChoirStaff, or for each staff ?

Yann