I am modeling a 30" system with A358 TP 304 welded pipe. This piping is made from rolled & welded plate. Do I check the "SeamWelded" box on the input spreadsheet???

"Yes", if you're running B31.3 and your temperature exceeds 950 degF.

But then you'll hve to enter a value for "W". Reference paragraph 302.3.5(e) for a definition of "W". You'll also have to enter "W" for the circumferential welds too, under SIFs.