If you have the space, or if the top of the head is below the static waterline, you really should have a vented loop. Its purpose is to prevent the plumbing from syphoning back into the boat.
Even if a boat's head is high enough to be above the waterline at rest, it will not be so when the head is on the leeward side in a breeze.
The joker valve that Waterline refers to will act as a check valve, but debris lodged in them will prevent a proper seal and they do weep more as they age. Even a small leak could sink your boat if you are not there to see the problem.
Another thing if you have a vented loop - remember there's more hose to clear so more pumping is required during a flush.