Now I'm not writing this post to comment on this "rule" but to talk about engagement rings in general.
Assuming that some of you are still waiting to get engaged - I know I started dreaming and planning my wedding before the ring was actually on my finger - there's no reason to just hope for a certain ring. You should feel comfortable explaining to your soon-to-be fiance what type of ring you want: stone, cut, setting. I would stay away from dictating carat size because if his budget won't allow a 4-carat ring then you are probably not going to get one and you should never say anything to make him feel uncomfortable about what he can or cannot afford.
If you don't want to just come out one day and say, "I want a brilliant cut diamond with baguette side stones on a platinum band", then you can show him rings that you like, or don't like, by commenting on rings you see in magazine, online and even on your friends hands.
Of course going shopping together is probably the safest bet to ensure that the ring you get is the ring you want, but if he does surprise you and you accept his proposal and you don't like the ring, just smile and enjoy the moment and even the next few days with this ring on your left hand. If after a few days you still don't like the ring style you can bring up the idea of changing it but I'd give it a chance. You don't want to hurt his feelings.