1) Door installation can be fast and easy if the rough opening is properly measured and framed. When replacing an existing door, the rough door opening should be 2-1/4" inches wider and 2" inch taller than the finished door dimensions, unless otherwise specified by the door manufacturer.
2) This extra spacing allows you to adjust and plumb your new prehung door assembly in case the opening isn't square.
3) Once the door is properly installed, the gap around the edges within the jambs at the top and sides will be equal.
4) The door should swing into the room against the nearest wall and take up as little living space as possible.
5) Closet doors are an exception: They should swing outward to maximize closet space.