depends on who you ask. If you drain, you know the fluid is out of there. If you leave in anti-freeze, you might still want to pull a plug quick to make sure the pink anti-freeze went through the block.
I'm anal, but in Northern WI, I run the boat to temp, drain the water from the block, then run the boat with anti-freeze, then drain everything completely. That way if I forgot to drain anything it is at least anti-freeze with a -50 burst proof protection.