First you have to answer the question if you want the sling only to help carry the rifle or also to help stead your shot.
Carry Slings: Don't use them but there are many newer options with flexible neoprene inserts which help decrease the felt weight of the rifle on your shoulder.
Shooting Slings: This is the real advantage of a sling. And for me I only use Ching Slings. The newer Safari Ching Sling made by Galco, allows a ching mounted on a two swivel mounts instead of the traditional three. I can move from carry to shooting position in seconds and get very steady hold off-hand and rock solid kneeling.
Enjoy your new Sako 300WSM; its a sweet rifle