(This mostly pertains to recreating a PK on an existing table.)
Is there any reason to have the PK index on an ever-increasing field (for example SERIAL, sequence or timestamp fed by clock_timestamp() at time zone 'UTC') be anything but fillfactor=100?
New records will always be added to the "lower right hand corner" of the tree, so having 20% empty space in the rest of the tree would just waste space (mainly buffers, but disk space could even start to add up on Very Big Tables).
Yes, at least that is what I gather from the advice on the CREATE INDEX page.