If its very cheap curtains-guess the bleach wouldn't be an issue.I do wash my curtains faily often-so the mildew won't form.Only time I use the bleach-is spray it on mildew that is trying to grow.One time caught it behind my washers-A spray of bleach from a spray bottle got rid of it-never seen it again.Don't like using bleach on my clothes and such-it is very powerful stuff and can cause damage.Guess we both live in mildew prone areas-the whole East coast can have the mildew problems from the humidity.