As far as I can say.. "usually" if you give credit or a linkback to the original post, than most authors are fine with it. If not, they can easily send you an email asking you to take it down.
Why not ask permission first? You don't go into a convenience store and swipe a candy bar thinking if the owner wants to be paid for it, he'll send you a message or call you on the phone. You don't go to a party at some acquaintance's house and take a DVD or CD, thinking if the guy really doesn't want you to have it, he'll find a way to get in touch with you.
Don't take things that don't belong to you. Period. You want something? Ask permission. Give the owner the respect they deserve and the opportunity to make the decision. If you just take something without asking, that's called stealing.