Our 100+ yr old plaster lathe walls from time to time develop hairline
cracks.

Googline, I see that some DIY articles recommend taping and others say
that for hairline cracks it is sufficient to dig it out and fill with
drywall or plaster patch.

Since taping takes more time and coats to feather out nicely, I would
like to know when (if ever) is it sufficient to just dig out the
crack and patch it without tape? (assuming that I want the crack not
to open up again)

Also, is it better to use plaster patching compound or drywall
compound? (the plaster patching compound drys much harder but is
therefore also harder to sand down)

Thanks