Yes, you must attach those "Y's" to the posts to stop all wobbling. If this is by a permit, the inspector will red tag this at once. Use the "y" concept like they use on telephone poles. Use long carriage bolts for fasteners. Do not even attempt to attach decking until the foundation is completely wobbble and bounce free.
Now also if these "Y"s are only on one side this will not stop the wobble, you need them going in alternate directions also. You need north and south and east and west supports.
The longer the "Y" the more stable the decking also.
I hope that you are working with code specs? If not and you have a permit, you might be removing the entire deck with a huge red tag. If not using a permit, then make sure its "right" again because of any law suits developing "IF" the deck comes down with people on it.