if my guitar has been set up in standard tuning... is it bad to leave in 1/2 step down?
Shouldn't give you any trouble, apart from some fret buzz maybe but that's easily fixed.
It would be fine. Guitar strings are pretty resilient.
it shouldn't I leave mine in drop D all the time
Leaving your guitar with slightly less tension then normal will do NO damage in any form.
It's perfectly fine. I leave mine in C standard.