It depends on how much you want to put out for it and how big you're expecting your place to get. Like Brian mentioned, there are several other extensions that people can snatch up and promote to possibly take some of your traffic, but it 'is' usually recommended to have the big three (I've seen the .info included in the recommendations, too). Personally, I'd say to start with your .com and once it starts getting built up, invest/insure yourself in the others if you really want them. You can certainly utilize them in different ways or simply reroute people to your place, but it helps prevent a few of the copycat possibilities.