It's a very brave man who relies only on the taper and nut tightness, particularly because the nut should not be excessively tight on a taper fitting. Castellated nut and split pin is my option. Never forget Murphy: "What can go wrong will go wrong".
The trouble with tab washers is that the always fit into the keyway in the prop. If you have a problem with the key - e.g. the previous owner fitted a brass key that dissolved away on my boat - then the tab washer is worse than useless. It forces the nut undone if the prop slips on the taper, as happened to me when the prop hit something underwater. There is a lot to be said for a split pin. At least the nut and prop are unlikely to fall off. Fortunately mine didn't. I was able to beach the boat soon after hearing a strange rattling noise from the prop shaft area! The key was like a miniature picture frame - all corners and no middle!
The split pin is there to stop the nut coming undone, which as Vye says, WILL happen sooner or later!! The taper on the shaft / prop distributes the load under power. ALSO - the split pin should be the same material as the shaft - prop - nut etc. In the 'old days' a yard would cast all the fittings for a boat at the same time - that way the content of the alloy was the same and there was less electrolysis