Safest course is yes...
Although a full holding tank won't sink the boat, it CAN make a major mess if it backs up into the toilet. And heeling can send the contents of even a half-full tank back down the head discharge line from the tank. A loop--not necessarily a vented one--in the line that's at least a foot above the waterline at any angle of heel prevents it from getting to the bowl...and a vented loop insures that it can't start a siphon in the unlikely event it does get that high. So I'd put one in if it were my boat. If you're also planning to plumb the system to be able to dump the tank at sea, you absolutely must put one between the thru-hull and the overboard discharge pump...and if there's only a tee or wye fitting in the tank discharge line, I'd replace that with a y-valve.
