I can't imagine why a hobbyist would not rinse GFO.
Rinsing with RODI is what I would do if using GFO. Load up the canister with GFO or GFO/Carbon mix to desired amount, grab a bucket of RODI, place it in, and run a couple of gallons through it. The hobbyist will see the GFO aftermath if only using GFO. Carbon, GFO combo is worse.
If the product isn't rinsed then it ends up in the sump. No filter media or control in the sump means it can make its way into the display. In both cases would not be a desired outcome. I would wager if clams are being kept the would not appreciate it.