Timing Marks on the 273 [66 Barracuda]

That's probably true with a factory mopar, without CAP, of this era.
However its always good practice to plug the distributor advance. On some setups there is a little bit of exposure of the vacuum port to manifold vacuum at idle. Also then its a good habit when dealing with vehciles that used manifold vacuum advance like an early 60s Kaiser Jeep.