Inam not speaking from experience, just using some logic.
Try putting a support under the center of the mast (even if it's just you lifting it). Do the ends lift
up off the rail? If so, then it has a curve in it and the rigging will have to straighten it as others have said. If lifting the middle just straightens it and the ends are still sitting on the bow & stern rails then there really isn't a sag in it.
As I stated, this is NOT from experience. just something I would do before I even began to worry about it.