i water at night all the time and never had a single issue, one thing for sure my worms love me for it!!
i think it may all depend upon temperature and thus rate of evaporation, im sure that it could be a problem if nights are cool for an extended period of time
this is to say if the water is sitting on the leaves too long and not evaporating, i have noticed that with how hot its been the water doent stick around long, the heat off the grownd prolly has much to do with that
also if you already have a large amount of the bacteria in the soil it could compound the issue, however i think its quite remote
if you have an issue deal with it accordingly thats how i do things
thanks your friend Joe