Well, it really depends if you like the 3D effects or not. If you enjoy 3D in a movie that probably shouldn't have it in the first place (in my opinion), go ahead. If you love the movie that much, go and watch it. The only thing really worth the 3D is the ability to see how close the Iceberg is in threeeeee-deeee!
There is need to watch the Titanic movie in 3D to get more thrill of the sinking scenes on the ship. 3D movies are known to be enhanced in illusion and depth perception. Visit http://www.titanicmovie.com/ for more details.