I suppose it is partly an ego thing, but knowing that the carbon print will still be looking like new a hundred years down the line, it would be nice to have future generations able to re-mat the print if needed. Or even re-wash the print if someone drools on it.
So I wait a few days to flatten my freshly printed carbon prints to allow the gel to harden, then flatten is a hot mount press (180 to 200F out of habit.) I transfer the image onto fixed out photo paper a size bigger than the tissue (11x14 for an 8x10 image). This extra paper around the print makes it easier to make my own corners and have the image area be nice and flat on the mat board.
Vaughn