Since I couldn't find any definitive information searching through this forum, I did online chat through Scion.com with one of their customer care staff.

"What is the recommended tire pressure for the 18" wheel/tire option for tC? The stock 17" wheel/tire is 32/29 but I was not able to locate any information on 18"."

He looked up the information and said Scion recommends 32/29 for the 18" option.

A rule of thumb seems to be that plus ones should get few extra psi from stock so I asked him again to make sure that information was for 18s. He confirmed that the information he found was for the 18s.

 

Moved to T&B...