Doors and Windows - exterior door installation

if you buy a door pre hung, before installation, and you see that it doesn't meet flush with the frame on the lower side, under the doorknob area, is that bad? will it correct when you install it? I bought a door pre-hung and it is doing this, wonder if to take it back? or if it will correct when we install it?

Prehung doors usually have these strips of wood stapled diagonally on the jambs in the cornors to keep it square during shipping. Sometime during shipping these strips come off and he jamb racks. Normally this will not cause a problem because when you install the door, you will plumb the jambs. However, if this was left like that for some time, like in storage, it might warp the jamb. If that portion of the jamb is warped, then take it back. The way to check it is to free the jamb and put a straight edge to the jamb and there should not be any gaps under at either end or middle of the straight edge.

When you install a pre-hung door, first raise the frame up on little blocks so that the bottom of the door clears any floor covering 1/2".
Then you start on the hinge side and overlap beveled shims from both sides (cedar shingle strips work well) behind the hinges, and use a level and a carpenter's square to get it plumb and square.
Work your way around the door. Don't sink your finishing nails all of the way until you have it perfectly square, plumb and level.
That should straighten any gaps, unless it is warped, as resercon said.
Good luck!
Mike