The 31.3 RFI seems clear, if your T1 is an operating temperature then it must be considered....

"B31.3 05-13065

In accordance with ASME B31.3-2004, when a piping system operates at multiple sets of coincident operating temperatures and pressures, and the system may at any random moment be loaded with an occasional load such as earthquake, wind, or pressure relief, should all coincident operating pressures and temperatures expected during operation be considered and shall the basic allowable stress used in para. 302.3.6 be based on the applicable temperature for each condition?

Reply Yes."