In other words, is it right to deliberately take the full phase length each time, even if you know what your moves are, just to pi55 someone off?
Well, it certainly isn't illegal and it's possibly impolite. But unethical or unfair? I don't think so.
Withholding orders is a legitimate, if fairly blunt, diplomatic tool and the phase length is the same for everyone in the game which everyone has signed up to voluntarily.
With that in mind, it seem churlish to complain just because someone is taking their own sweet time.
I think that if you are the last one waiting to order and you know that you can keep the game ticking on by finalising your orders, then it's a good/nice thing to do so. And if you're using that time constructively to try and negotiate a particular move then by all means take all the time that you can.
But if you suspect that someone else is deliberately slowing the game, then I feel that becomes a personal challenge to you to respond to. Use your own diplomatic skill, either to confirm that they are actively negotiating (and find out what), try to speed them up, or use the extra time to negotiate an alliance against them!