Plussing and tiering are not related.
You can plus an item and then tier or tier then plus.
There are players who believe the higher tier something is above its' base tier, the more likely the plussing jewels might fail. These players suggest getting an item plussed before tiering so that you use less jewels.
Sometimes, with the amount of jewels I go through plussing an item that's already statted, I suspect they could be right...