> ( ... my question applies to any device with trim)
There's no one general "right answer" because sometimes the thing which is supposedly just a "trim" control is actually the overall sensitivity control for the input stage; other times it really is just a small variable boost later on in the circuit, and is mainly there to help you optimize your recording levels, as you might expect.
You need to set good levels without overloading any stage of a recorder (input, intermediate, final, whatever), but the meters usually only tell you what's happening at the final stage of recording (i.e. at the input of the A/D converter). That's always a problem, and with different sets of equipment it has to be worked out differently. Sometimes you have to pad a signal before sending it into the recorder in the first place (like when people get clipping even when their peaks are only at -10 dB).
--best regards