Is there a widely followed standard or convention in the audio industry
on whether amps should have coupling caps on their input jacks (to
keep stray DC bias from tuners or CD players or tape machines from
goofing up bias on tube grids)?. Or is it expected that all audio sources
are not to have any stray DC bias riding on the audio? And that there
should never be any DC bias leaking from the amp to the source?

If there is a convention, then one could avoid having a coupling cap
inside the source feeding the output jacks, only to feed into a 2nd
coupling cap in the amp.