If you just need to make sure a MULTILINESTRING is returned, use ST_Multi:

ST_Multi — Returns the geometry as a MULTI* geometry. If the geometry is already a MULTI*, it is returned unchanged.

http://postgis.refractions.net/docs/ST_Multi.html

If you just need to make sure a MULTILINESTRING is returned, use ST_Multi:

ST_Multi — Returns the geometry as a MULTI* geometry. If the geometry is already a MULTI*, it is returned unchanged.

http://postgis.net/docs/ST_Multi.html