it shouldnt hurt anything
its just the jiuce from a bean
so, it does add amino acids, minerals etc...
and yes it can lower the PH (coffee itself)
It shouldnt hurt, even if used daily
but, i would keep a close eye on new growth, leaf color etc...
and maybe do it for a few days at a time, then wait a week to see if there are any changes.
(and let the soil dry out obviously)
As far as grounds
i cant say enough about them.
worms love em, and worms aerate the soil, and leaves free fertilizer behind.
i get about 80lb a week from Starbucks, sometimes more.
i sprinkle them on the ground, put in compost piles
and put in my worm bin.
you can wet them down with a mix of fish emulsion, and molasses
then, bacteria and fungi go to work right away
breaking them down into beautiful stuff.
sometimes i mix with dry lawn clippings too.
doing either, you have to make sure its aerobic.
they have to be turned a lot, or bad fungi will take over.
the dry grass seems to keep air in the pile and make it easier to turn.