Each of the Shardholders reflect the persona of their shard.
My suggestion was, since Sazed holds two shards, he isn't either Preservation or Ruin. Since the shards could still be separated from Sazed (evidenced by death of Leras and Ati, Aona and Skaze), you wouldn't remove them from the total-shard count.
If you were to consider yourself different from Preservation and Ruin, I'd assume you'd simply have a new name for yourself - thus the 17th is born.
I still think its feasible that Sazed has a role to play in the group "seventeeth shard". If people have started world-hopping, why couldn't Sazed be involved? With his thirst for knowledge and now immortality/time to play with he could have hopped worlds or instructed others to look for people with information on shards. (Note: this is more of a theory considering there definitely isn't enough information out there on the events after Hero of Ages or the seventeeth shard other than a few references.)