Tekin's design for keeping the ESC small is to use the posts (and by extension your copper wires) as a direct heat sink. The posts are directly connected to the FET transistors of the ESC, so they can remove heat the fastest and they get airflow over them in normal use. No, they aren't there to keep you from burining the board as you won't burn anything at the temps your iron will get to, it's there to disapate heat. Go ahead and heatshrink them up, I'm not your boss, and you'll probably get away with it in stock racing, but do know that Tekin refuses warranty claims on ESC's that arrive with heat shrink on the posts.