Aerodynamically it makes sense to have swept back wings below M 1.0
(in the transsonic range) to avoid the onset of Mach effects on certain
parts of the airframe.
Once you are above M 1.0-1.1 that does not matter anymore as everything
is supersonic anyway.
Furthermore there are even people around claiming that the drag of a
straight wing at full supersonic speeds (> M 1.3-1.5) is even lower than
that of a swept back wing - however I do not fully believe that.
EDML