Well, it all depends, really. Both bride and groom can get married due to family obligations or just personal interest, which would affect if they know each other or not.
The reason Edmure didn't know his bride is because Lord Walder chose one among his own daughters and he was obliged to accept, but usually that is not the case. Most people before arranging a betrothal demand guarantee that the other's candidate is healthy, or has some value, most times inheritance.
Sometimes the head of a noble house may allow a member to marry just for love, or marry himself for such reasons. Sometimes two friends  simply want to join houses so they marry their children.