The taboo could be extended to Holders in general because they -technically- are in line for the Throne.
As I've stated elsewhere, though, for all practical intents and purposes most probably only need to follow it by choice, because the assassinations that would need to take place for them to even be considered "in the running" are of such a scale and level of coordination that borders on the ridiculous.
I expect Holders that do abide by it are looked upon more kindly, but I'm not sure it's a gamebreaker for those much lower on the food chain, let alone worth the time of fussing over.