leaving alternator breakers on all the time

Hi -

My PA23-250 POH uses strong wording to insist that whenever the
engines are running, the alternator breakers should be on. This
contradicts some checklists for this plane (and those of a few other
single-engine Cessnas) that suggest that those breakers be open during
engine start and after shutdown. On the twin, one checklist includes
an alternator load check during runup that includes opening one then
the other breaker, to ensure that the electrical load can be carried
by either unit - that sounds valuable but appears to go against the POH.

Can someone offer an explanation of the relative wisdom of two
approaches?

- FChE