For fishing the beach you are right on in fact I use 15 lb.
Off the rocks I use 30lb as the Tailor always fight harder and you will always run into those big greenbacks.
Wazza is right there are some bigs ones around and I find wire is the only thing to stop them. Over the last 18 months I have moved from 40 to 60 to 80 lb leaders and finally nylon coated wire and it is amazing what they have done to the wire.
It also depends upon your bait as to what setup.