Trim Necessary with Assist Enabled (not OFF)?

Assuming one has performed the "Quick Trim Setting" to define "level" flight,
then assuming a properly setup model, is trim necessary; i.e., won't the Assist
compensate for small deviations in trim?

Specifically, the manual implies that "heading hold" should not have trims applied, but what about "normal" mode?
Why wouldn't the set attitude be maintained by the Assist Rx?

These questions are based upon my experiences flying the Eagle Tree Systems Guardian and enabling the Guardian
even for maiden flights; trim isn't absolutely necessary assuming reasonable setup.

Michael